GIFT   OF 

MICHAEL  REESE 


A 


HANDBOOK  OF  ROCKS, 


FOR 


WITHOUT  THE  MICROSCOPE. 


BY 

JAMES    FURMAN   KEMP,  A.B.,  E.  M., 

PROFESSOR  OF  GEOLOGY  IN  THE  SCHOOL  OF  MINES,  COLUMBIA  UNIVERSITY, 

NEW  YORK. 


GLOSSARY  OF  THE  NAMES  OF  ROCKS 


AND    OF    OTHER 


LITHOLOGICAL   TERMS. 


PRINTED  FOR  THE  AUTHOR. 
NEW   YORK. 

1896. 


&  ft  4-3 

COPYRIGHT,  J.  F.  KEMP,  1896. 


THE  NEW  ERA  PRINTING  HOUSE, 
LANCASTER,  PA. 


PREFACE. 


The  clear  presentation  of  the  subject  of  rocks  to  beginners  is 
not  an  especially  simple  undertaking.  The  series  of  objects  is  ex- 
tremely diverse,  and  many  unrelated  processes  are  involved  in 
their  production.  In  order  not  to  confuse  and  bewilder  students, 
the  teacher  must  emphasize  the  intelligible  points  and  the  recog- 
nizable characters,  avoiding  alike  distinctions  that  have  their  chief 
foundations  in  past  misconceptions,  such  as  the  time  element  in 
the  classification  of  igneous  rocks,  or  that  require  microscopic 
study  to  substantiate  them.  In  the  following  pages  the  attempt 
has  been  made  to  avoid  these  difficulties,  and  only  to  mention  and 
emphasize  the  characters  that  a  beginner,  properly  equipped  with 
the  necessary  preliminary  training  in  mineralogy,  can  observe  and 
grasp.  £  ,,  ^  :. 

Some  years  of  annually  going  over  this  ground  have  convinced 
the  writer  that  for  this  purpose  we  are  not  likely  to  reach  a  more 
serviceable,  fundamental  classification  than  the  time-honored  one 
of  Igneous,  Aqueous  (or  Sedimentary)  and  Metamorphic  rocks. 
They  furnish  not  alone  convenient  central  groups,  but  also  prepare 
the  student  for  subsequent  geological  reading.  With  the  Aqueous 
have  been  placed  the  Eolian  as  a  similar,  although  very  minor  di- 
vision, so  that  fire,  water  and  air,  the  ancient  elementary  agents, 
are  emphasized  in  their  work  upon  the  earth,  and  the  fundamental 
classification  is  based,  as  it  should  be,  on  method  of  origin.  The 
only  illogical  step  involved  is  the  placing  of  the  breccias  together 
with  the  sediments,  but  breccias  are  so  subordinate  and  go  so  con- 
veniently with  conglomerates  that  it  has  been  done. 

The  igneous  rocks  are  the  ones  that  present  the  greatest  diffi- 
culties to  the  learner.  In  the  following  pages,  after  a  preliminary 
exposition  of  principles,  the  very  minor  group  of  the  volcanic 
glasses  is  first  taken  up,  because  it  is  the  simplest  and  because  it 
illustrates  cooling  from  fusion  most  forcibly.  Passing  then  through 
the  felsitic  and  porphyritic  to  the  granitoid  textures,  rocks  of  in- 
creasing complexity  are  one  after  another  attacked.  Analyses 
have  been  freely  used  to  illustrate  the  chemical  differences  of  mag- 
mas, because  in  no  other  way  can  the  varieties  be  fundamentally 


IV  PREFACE. 

described.     Within  fairly  narrow  limits  the  chemical  composition 
of  the  magma  establishes  the  mineralogy  of  the  rock. 

The  Aqueous  and  Eolian  rocks  are  not  difficult  to  understand. 
The  Metamorphic  are  in  many  respects  the  most  obscure  of  all, 
but  it  is  hoped  that  enough  varieties  have  been  selected  and  em- 
phasized to  serve  for  field  use  and  for  the  reasonably  close  deter- 
mination of  the  great  majority  of  those  that  will  be  met  in  Nature. 

Many  names  will  be  encountered  in  geological  reading  that  are 
not  mentioned  in  the  book  proper.  To  explain  them  and  to  avoid 
confusing  the  main  text  with  unessential  matter,  they  have  been 
compiled  in  a  Glossary.  Practically  all  the  names  for  rocks  will 
be  found  there,  and  some  related  geological  terms.  The  chief 
guide  in  its  preparation  has  been  the  index  of  Zirkel's  great  Le/ir- 
buch  der  Petrographie ,  but  not  a  few  American  terms  are  intro- 
duced, which  are  not  in  it  nor  in  Loewinson-Lessing's  Petrpgrapli- 
isches  Lexikon,  to  which  the  writer  is  also  greatly  indebted. 
Other  works,  English,  French  and  American,  have  likewise  been 
at  hand.  One  only  needs  to  compile  a  glossary  to  appreciate 
what  numbers  of  unnecessary  and  ill-advised  names  for  rocks  bur- 
den this  unfortunate  branch  of  science,  and  to  convince  one  that 
the  philological  petrographer  comes  near  to  being  the  enemy  of 
his  kind. 

So  far  as  possible,  technical  words  of  classic  derivation  have 
been  avoided  in  the  main  work  in  favor  of  simple  English,  and  for 
the  rocks  described,  American  type  have  been  especially  sought 
with  which  to  illustrate  the  different  species,  because  they  are  more 
significant  and  accessible  to  readers  on  this  side  of  the  ocean.  The 
text  except  the  glossary,  appeared  as  a  series  of  papers  in  the 
SCHOOL  OF  MINES  QUARTERLY  during  1895-96. 

J.  F.  K. 
AUGUST,  1896. 


TABLE  OF  CONTENTS. 


Preface iii 

Abbreviations vi 

Errata vii 

CHAPTER  I. — Introduction.  Rock-forming  Minerals.  Princi- 
ples of  Classification I 

CHAPTER  II. — General  Introduction  to  the  Igneous  Rocks  .       12 

CHAPTER  III. — The  Igneous  Rocks,  continued.  The  Glasses. 
The  Rocks  whose  chief  feldspar  is  orthoclase.  The  Pho- 
nolites  and  Nepheline-Syenites 20 

CHAPTER  IV. — The  Igneous  Rocks,  continued.  The  Dacites, 

the  Andesites  and  the  Rocks  of  the  Basalt  Group  ...  39 

CHAPTER  V. — The  Igneous  Rocks,  continued.  The  Diorites, 
Gabbros,  Pyroxenites  and  Peridotites.  Ultra-basic  Igne- 
ous Rocks 47 

CHAPTER  VI. — Remarks  in  review  of  the  Igneous  Rocks  .    .       54 

CHAPTER  VII. — The  Aqueous  and  Eolian  Rocks.  Introduc- 
tion. The  Breccias  and  Mechanical  Sediments  not  Lime- 
stones    58 

CHAPTER  VIII. — Limestones.  Organic  Remains  not  Lime- 
stones. Rocks  Precipitated  from  Solution.  Determina- 
tion of  the  Aqueous  and  Eolian  Rocks 70 

CHAPTER  IX. — The  Metamorphic  Rocks.  Introduction.  The 

Rocks  Produced  by  Contact  Metamorphism 84 

CHAPTER  X.— The  Metamorphic  Rocks  continued.  The  Rocks 
Produced  by  Regional  Metamorphism.  Introduction. 
The  Gneisses  and  Crystalline  Schists 93 

CHAPTER  XL — The  Metamorphic  Rocks,  continued.  The 
Rocks  Produced  by  Regional  Metamorphism.  The  Quartz- 
ites  and  Slates.  The  Crystalline  Limestones  and  Dolo- 
mites. The  Ophicalcites,  Serpentines  and  Soapstones  .  106 

CHAPTER  XII. — The  Metamorphic  Rocks,  concluded.  The 
Rocks  Produced  by  Atmospheric  Weathering.  ,  The  De- 
termination of  the  Metamorphic  Rocks 116 

Glossary 121 

Index  .  I71 


, 

XJNIVERSITT. 


A    HAND    BOOK   OF;,R<3CKS. 

FOR  USE  WITHOUT  THE  M?CR6s*sOBEi f  ','*'8 


i 


CHAPTER  I. 

INTRODUCTION.     ROCK-FORMING  MINERALS.     PRINCIPLES 
OF  CLASSIFICATION. 

A  rock  may  be  best  defined  as  any  mineral  or  aggregate  of  min- 
erals that  forms  an  essential  part  of  the  earth.  The  word  mineral 
is  used  because  this  is  our  most  general  term  for  all  inanimate  na- 
ture, and  while  the  lifeless  remains  of  organisms  often  contribute  in 
no  small  degree  to  rocks,  no  rock  is  made  up  of  those  which  are 
still  alive.  In  instances  a  single  mineral  forms  a  rock,  but  among 
minerals  this  is  the  exception.  By  far  the  greater  number  are  in 
such  small  amount  that  they  cannot  properly  be  considered  rocks. 
Rock-salt,  ice,  calcite,  serpentine,  cemented  fragments  of  quartz, 
kaolin  and  a  few  others  are  in  sufficient  quantity,  but  the  vast  ma- 
jority of  rocks  consist  of  two  or  more.  The  condition  that  a  rock 
should  form  an  essential  part  of  the  earth  is  introduced  to  bar  out 
those  minerals  or  aggregates  which,  though  important  in  them- 
selves, are  none  the  less  insignificant  as  entering  into  the  mass  of 
the  globe.  Thus  the  sulphide  ores,  while  locally  often  in  con- 
siderable quantity,  when  broadly  viewed  are  practically  neglectable. 
Yet  this  is  somewhat  arbitrary  and  there  are  single  minerals  and 
aggregates  that  may  properly  give  rise  to  differences  of  opinion. 
The  following  pages  err,  if  at  all,  on  the  side  of  demanding  that 
the  amount  should  be  large.  A  rock  must  also  have  an  individual 
haracter,  sufficient  to  establish  its  identity  with  satisfactory  sharp- 
ness. The  species  cannot  be  marked  off  with  the  same  definition 


2  A  HANDBOOK  OF  ROCKS. 

as  in  plants,  animals  or  minerals,  and  there  is  here  again  reason- 
able opportunity  for  differences  of  opinion  as  to  the  limits  which 
should  be  set,  some  admitting  of  finer  distinctions  and  greater  multi- 
plicity of  species  than  others ;  but  after  all  has  been  said,  there  should 
be  a  well  marked  individuality  to  each  rock  species  such  that  any 
careful  and  qualified  observer  may  readily  see.  Too  great  refine- 
ments and  too  minute,  subdivisions  ought  to  be  avoided.  The  de- 
termining coua'iti-anis  of  species  will  be  taken  up  at  greater  length 
when  the  preliminaries  *cf  classification  have  been  set  forth,  but  it 
inu-srb;;  appreciated -thet  the  point  of  view  is  also  a  most  important 
factor.  Thus  if  one  is  studying  the  geology  of  a  district  with  close 
accuracy,  and  is  tracing  out  the  history  and  development  of  its 
rocks  with  microscopic  determinations  and  descriptions  of  min- 
erals and  structures  which  may  be  minute,  finer  distinctions  will 
naturally  be  drawn  than  those  that  suggest  themselves  to  one  who 
is  engaged  in  ordinary  field  work  or  in  mining  or  engineering 
enterprises.  It  is  for  tne  latter  class  that  these  pages  are  prepared 
and  throughout  the  descriptions  and  classification  here  given,  the 
necessary  limitations  and  the  practical  needs  of  such  observers  are 
always  kept  in  mind.  Textural  and  mineralogical  distinctions  are 
alone  emphasized  where  easily  visible  on  a  specimen,  although 
never  made  contradictory  of  principles  of  origin  and  classification 
that  could  be  carried  to  greater  length  and  subdivision. 

Rocks  embrace  matter  in  a  great  variety  of  structures  and  con- 
ditions. While  in  general  we  picture  them  to  ourselves  as  solid, 
yet  under  the  terms  of  our  definition,  we  have  no  logical  right  to 
bar  out  liquids  or  even  gases.  The  physical  condition  may  vary  with 
ordinary  temperatures.  Thus  we  cannot  reject  ice  as  an  extremely 
abundant  and  important  rock,  and  yet  its  solid  condition  results 
from  water  with  a  moderate  loss  of  heat,  and  at  ordinary  tem- 
peratures the  same  molecules  may  be  in  a  liquid  or  gaseous 
state.  All  that  we  know  of  volcanoes  indicates  that  liquid,  molten 
magmas  exist  for  long  periods  deep  in  the  earth,  yet  they  are 
none  the  less  rocks  because  of  their  liquidity.  In  general,  how- 
ever, rocks  are  solid,  and  gases  or  liquids  (except  water)  de- 
serve no  further  attention.  In  texture  rocks  may  be  loose  and  in- 
coherent as  in  sand,  gravel,  volcanic  dust  and  the  like,  or  they 
may  be  extremely  dense,  hard  and  solid,  as  in  countless  familiar 
examples.  This  solidity  or  massiveness  has  its  limitations,  for  all 
observation  and  experience  show  that  what  are  apparently  solid 


INTRODUCTORY.  3 

masses  are  really  broken  up  by  multitudes  of  cracks  into  pieces 
of  varying  size.  All  quarries  and  mines  have  these,  and  they  may 
aid  or  annoy  the  operators  according  to  the  purposes  of  excava- 
tion. They  will  again  be  referred  to  at  length.  Rocks  are  also 
in  all  cases  permeated  with  minute  pores  and  spaces  that  admit  of 
the  penetration  of  water  and  other  liquids,  especially  if  under  pres- 
sure. These  are  important  factors  in  terrestrial  circulations. 

THE  CHEMICAL  ELEMENTS  IMPORTANT  IN  ROCKS. 
The  chemical  elements  really  important  in  rocks  are  compara- 
tively few,  and  are  those  which  are  most  widespread  in  nature. 
The  best  estimate  that  has  been  made  is  that  of  F.  W.  Clarke,  in 
Bulletin  78,  of  the  U.  S.  Geological  Survey,  pp.  34-43.  The  crust 
to  ten  miles  below  sea  level  and  the  air  and  the  ocean  are  embraced. 
The  composition  of  the  solid  crust  is  reached  by  averaging  up 
analyses  of  igneous  and  crystalline  rocks,  880  in  all;  321  from 
the  United  States,  75  from  Europe,  486  from  all  quarters.  Ig- 
neous rocks  being  the  source  of  all  the  others,  furnish  the  best 
data  for  the  general  chemistry  of  the  globe.  The  composition  of 
the  ocean  is  then  averaged  in  with  that  of  the  rocks  on  the  basis  of 
7%  for  the  former  and  93%  for  the  latter,  with  a  further  addition 
of  0.02%  for  the  nitrogen  of  the  atmosphere.  Other  ingredients, 
as  the  oxygen  of  the  air  are  less  than  0.01%  and  are  neglected. 

O  49.98 

Si  25-3° 

Al  7.26 

Fe  5.08 

Ca  3.51 

Mg  2.50 

The  remaining  elements  may  be  omitted  in  this  connection, 
although,  as  a  moment's  reflection  will  show,  they  include  all  the 
common  metals  except  iron  and  manganese. 

There  is  good  ground  for  believing  that  toward  the  centre  of  the 
earth  the  metallic  elements  become  much  more  abundant,  and 
that  near  the  centre  some  of  the  heaviest  known  are  in  excess,  but 
these  inferences,  however  well-based,  concern  materials  far  beyond 
actual  experience,  and  of  no  great  moment  in  this  connection.  As 
regards  rocks  we  have  to  deal  with  the  outer  portions  of  the  globe, 
to  which  we  are  accustomed  to  refer  as  the  crust.  This  term  is  not 
meant  to  indicate  anything  as  to  the  condition  of  the  interior,  but 
merely  its  exterior  as  contrasted  with  the  inner  parts. 


Na 

2.28 

P 

0.09 

K 

2.23 

Mn 

0.07 

H 

0.94 

S 

0.04 

Ti 

0.30 

Ba 

0.03 

C 

0.21 

N 

O.O2 

Cl,  Br 

O.I5 

Cr 

O.OI 

4  A  HANDBOOK  OF  ROCKS. 

The  chemical  elements  above  cited  are  combined,  except  per- 
haps in  volcanic  glasses,  in  the  definite  compounds  that  form 
mineral  species.  These  compounds  change,  more  or  less,  in  the 
course  of  time,  under  the  action  of  various  natural  agents,  chief  of 
which  are  water,  carbonic  acid  and  oxygen,  but  at  any  particular 
stage,  however  complex  the  rock  may  be,  it  is  made  up  of  definite 
chemical  compounds,  though  we  may  not  be  able  to  recognize 
them  all.  The  most  important  compounds  are  not  numerous  and 
are  practically  limited  to  the  following:  silicates,  oxides,  carbo- 
nates, sulphates,  chlorides,  and  of  far  inferior  moment  phosphates,, 
sulphides,  and  one  native  element  graphite. 

As  a  broad  conception  in  speaking  of  these  compounds  it  is  in 
many  respects  advantageous  to  have  the  igneous  rocks  primarily 
before  our  minds,  because  as  stated  above  they  are  the  sources  of 
the  others.  In  taking  up  the  minerals  the  purpose  here  is  to  em- 
phasize their  chemical  composition  and  relative  importance,  not  to 
describe  them  as  would  be  done  in  a  text-book  on  mineralogy  so 
as  to  enable  a  student  to  recognize  them,  for  such  preliminary 
knowledge  is  here  assumed.  Our  purpose  is  to  make  prominent 
the  chief  chemical  compounds  entering  into  the  earth,  and  to  pre- 
pare the  way  for  a  true  conception  of  the  range  and  relations  of  its 
constituent  rocks. 

THE  SILICATES. 

THE  silicates  are  grouped  as  follows :  the  feldspars  and  feld- 
spathoids;  the  pyroxenes;  the  ampjiiboles;  the  micas;  olivine. 
The  last  four  groups  are  often  collectively  called  the  ferro-magnesian 
silicates.  Zircon  and  titanite  conclude  the  list  of  those  important 
in  igneous  rocks.  In  addition  there  are  a  number  of  others  that 
are  specially  characteristic  of  altered  or  metamorphosed  rocks,  viz  : 
epidote,  scapolite,  garnet,  tourmaline,  topaz,  andalusite,  cyanite, 
fibrolite  or  sillimanite,  and  staurolite.  Finally  a  few  hydrated  sili- 
cates complete  the  list. 

THE  FELDSPARS  and  their  related  minerals  are  all  double  silicates 
of  alumina  and  an  alkali  or  an  .alkaline  earth  or  both.  We 
speak  of  them  as  alkali-feldspar,  potash- feldspar,  soda-feldspar, 
lime-soda  feldspar,  etc,  based  on  this  fact.  They  are  generally 
grouped  as  orthoclase,  representing  monoclinic  feldspar  with  its 
two  cleavages  at  right  angles  (hence  the  name),  "ami  as  plagioclase* 
or  triclinic  feldspar,  with  oblique  cleavages,  and  one  striated  cleav- 
age plane.  Orthoclase  is  chiefly"  i€A4S^Og-,-^5ur~N^— replaces 


INTRODUCTORY.  5 

more  or  less  of  the  K,  without  affecting  the  crystal  system.  Suffi- 
cient amounts  of  soda  are  however  capable  of  changing  the  system 
to  triclinic  and  the  feldspar  is  called  anorthoclase.  Microcline  is 
also  a  triclinic  variety  of  potash  feldspar,  with  a  cleavage  angle 
slightly  less  than  a  right  angle,  but  with  peculiar  and  character- 
istic optical  properties,  which  are  chiefly  of  moment  in  micro- 
scopic work.  The  clear,  unclouded  orthoclase  of  the  later  volcanic 
rocks  is  often  called  sanidine.  It  does  not  differ  essentially  from 
the  orthoclase  of  the  older  rocks,  and  the  distinction  based  on 
geological  age  is  obsolete,  but  as  the  terms  are  still  used  in  the 
literature  of  the  subject  it  is  well  to  understand  them. 

The  plagioclase  feldspars  embrace  a  practically  unbroken 
series  from  pure  soda-alumina  silicate  in  albite,  NaAlSi3O8,  to 
pure  lime-alumina  silicate,  anorthite,  CaAl2Si2O8.  Various  mix- 
tures of  these  two  molecules  give  the  intermediate  species,  but  the 
two  on  which  special  stress  is  ordinarily  placed  are  oligoclase, 
with  soda  in  excess  and  hence  called  soda-lime  feldspar,  and  labra- 
dorite  with  lime  in  excess  and  hence  called  lime-soda  feldspar.  If 
we  represent  the  orthoclase  molecule,  KAlSi3O8  by  Or ;  the 
albite  molecule,  NaAlS53O8  by  Ab,  and  the  anorthite,  CaAl2Si2O8 
by  An,  all  the  intermediate  feldspars  can  be  algebraically  ex- 
pressed. Thus  anorthoclase  lies  between  Ab2Orlf  and  Ab^Or^ 
Albite  embraces  those  from  Ab  through  Ab^n^  oligoclase,  AbgAn^ 
through  Ab2Anx;  (the  intermediate  mixtures  Ab3An2  through 
Ab4An3  are  called  andesine);  labradorite  includes  A^A^  through 
AbjAn^  bytownite  AbtAn3 — A^An^  anorthite  AbxAn8  to  An. 
This  conception  of  feldspars  as  isomorphous  mixtures  of  molecules  is 
a  very  valuable  one  and  by  determining  specific  gravity,  optical 
properties  and  chemical  composition,  one  or  all,  the  different 
members  can  be  identified.  Practically,  however,  in  the  ordinary 
determination  of  rocks,  aside  from  microscopic  work  we  are  forced 
by  the  difficulty  of  distinguishing  the  intermediate  varieties,  into 
the  general  use  of  orthoclase  and  plagioclase,  and  we  rely  on  the 
presence  or  absence  of  the  striations  peculiar  to  the  basal  cleavage 
of  the  latter  in  distinguishing  between  the  two,  but  of  course  ex- 
perience and  familiarity  with  the  general  characters  and  associa- 
tions of  minerals  in  rocks  often  enables  one  to  determine  very 
closely  the  minor  varieties.  We  would  naturally  look  for  ortho- 
clase, albite  and  oligoclase  in  acidic  rocks  or  those  high  in  silica, 
while  in  basic  rocks  we  would,  expect  those  near  the  anorthite  end. 


6  A  HANDBOOK  OF  ROCKS. 

All  the  feldspars  have  very  similar  crystal  forms  when  these  are 
developed,  as  they  occasionally  are  in  rocks.  When  they  are 
small  and  irregularly  bounded,  cleavage  faces  should  be  sought 
out  and  examined  with  a  pocket  lense.  It  is  interesting  to  note 
that  only  in  igneous  rocks  do  we  obtain  crystals  uniformly  devel- 
oped on  all  sides,  for  only  in  a  fused  magma  do  they  swim  and 
grow  without  a  fixed  support. 

The  word  feldspar  is  spelled  by  English  writers  "  felspar,"  but 
among  Americans  the  more  correct  form,  based  on  the  etymology, 
is  employed,  following  the  German  original  "  Feldspath." 

FELDSPATHOIDS.  With  the  feldspars  are  placed  two  other  im- 
portant and  closely  related  minerals,  nepheline  and  leucite,  to  which 
may  also  be  added  one  that  is  quite  rare,  melilite.  Nepheline  is  an 
hexagonal  soda-alumina  silicate  4Na2O,4Al2O3,9SiO2,  in  which  some 
of  the  Na2O  is  replaced  by  K2O  and  CaO.  It  appears  in  a  subordi- 
nate series  of  igneous  rocks  that  are  rich  in  soda.  Leucite  is  an  iso- 
metric potash  silicate,  K2O,Al2O3,4SiO2,  with  a  little  Na2O  replacing 
part  of  the  K2O.  It  appears  as  an  important  rock-making  mineral 
in  the  igneous  rocks  of  ten  or  fifteen  localities  the  world  over,  and  is 
therefore  of  very  limited  distribution.  Melilite  is  an  extremely  basic 
lime-alumina  silicate,  i2CaO,2Al2O3,9SiO2,  and  appears  in  a  few 
rare  basalts. 

Reference  may  also  be  made  to  sodalite,  nosean  and  haiayne 
which  are  occasionally  met,  but  which  are  chiefly  of  microscopic  in- 
terest. 

The  feldspars,  together  with  the  feldspathoids  nepheline  and 
leucite,  are  the  most  important  of  the  rock-making  minerals  in 
their  relations  to  the  classification  of  rocks. 

-  In  order  to  have  a  standard  series  of  analyses  with  which  to 
compare  those  of  rocks  later  given,  the  following  table  is  inserted 
of  theoretical  feldspars  and  feldspathoids.  The  relative  amounts 
of  the  several  oxides  will  suggest  the  extent  to  which  the  mole- 
cules are  present  in  any  rock  whose  analysis  is  known : 

OR  AB  AN 

ORTHOCLASE         ALBITE  ANORTHITE  NEPHELINE  LEUCITE  MELILITE 

KAlSi308     NaAlSi308     CaAl2Si2O8         Na8Al8Si9O34     KAlSi2O6       Ca12Al4Si9O36 

SiO3  64.7  68.6            43.1                45.0                55.0                38.1 

A12O3  18.4  19.6            36.8                 34.3                23.5                14.5 

K2O  16.9 21.5 

Na2O  .  .         1 1.8  .        .        .        .       20.7 

CaO  .         .         .         .         .     20.  i 47.4 


INTRODUCTORY.  7 

Recalling  what  has  been  said  about  the  replacement  of  the  al- 
kalies by  one  another,  and  that  we  never  meet  any  of  these  min- 
erals chemically  pure,  according  to  the  formulas  above  given,  and 
making  suitable  allowance  for  this  replacement,  we  may  still  ap- 
preciate that  orthoclase  and  albite,  being  high  in  silica,  favor 
acidic  rocks,  and  the  others  being  low  in  silica,  basic  ones  ;  that 
nepheline  implies  a  magma  rich  in  alumina  and  soda,  leucite  one 
rich  in  potash,  and  melilite  one  low  in  silica  and  alumina,  but  high 
in  lime. 

THE  PYROXENES  and  the  AMPHIBOLES  are  best  described  to-V 
gether.  Each  embraces  a  series  of  compounds  of  the  same  chemical 
composition,  differing  only  in  physical  and  optical  properties.  As 
the  table  shows  they  vary  from  magnesia  silicate  through  a  series 
of  lime  and  lime-alumina  silicates,  with  an  iron  silicate  generally 
present.  All  the  pyroxenes  have  a  prismatic  cleavage  of  nearly 
90°  (87°  10'  or  thereabouts),  while  the  amphiboles  cleave  along  a 
prism  of  nearly  120°  (124°  I  i'). 

COMPOSITION. 


rn  •» 
-  JV2  J- 


PYROXENE. 

AMPHIBOLE.                 SYSTEM. 

Enstatite 
Bronzite 
u  Hypersthene 

Anthophyllite    I  Orthorhombic 

Diopside 
Malacolite 
j  (Diallage) 
^Augite 

Tremolite         "1 
Actinolite 

Hornblende      [>  Monoclinic 

Acmite 
Aegirine 

Arfvedsonite    | 

\  FeOSiO 

CaMgSi2O6 

CaFeSi2O6 

MgAl2Si06 

MgFe2Si06 

FeAl2SiO6 

NaFeSi2O6 


Under  the  orthorhombic  pyroxenes  enstatite  has  least  of  the 
molecule  FeOSiO2,  i.  e.  FeO  less  than  $%  ,  bronzite  has  FeO 
less  than  14%  and  hypersthene  the  higher  values.  The  increase 
brings  about  a  darker  color  and  changed  optical  properties.  The 
orthorhombic  pyroxenes  are  much  less  frequent  than  the  mono- 
clinic,  but  are  of  wide  distribution,  especially  hypersthene.  The 
orthorhombic  amphiboles  are  of  minor  importance  and  are  but  sel- 
dom met. 

The  light-colored  monoclinic  pyroxenes  are  almost  pure  lime 
magnesia  silicates,  and  are  called  diopside.  They  are  chiefly  found 
in  crystalline  limestones.  As  iron  increases,  they  pass  into  malaco- 
lite,  which  may  also  contain  small  amounts  of  the  aluminous  mole- 
cules. Neither  of  these  pyroxenes  is  of  special  abundance  as  a  rock 
maker.  When  pinacoidal  cleavages  around  the  vertical  axis  appear 


8  A  HANDBOOK  OF  ROCKS. 

in  addition  to  the  prismatic  ones  in  pyroxenes  of  the  general  compo- 
sition of  malacolite  they  are  called  diallage  and  are  important  in  some 
igneous  rocks.  But  the  chief  rock-making  pyroxenes  are  the  dark 
aluminous,  ferruginous  ones,  which  are  called  augite,  and  these  are 
among  the  most  important  of  all  minerals  in  this  connection.  The 
igneous  rocks  rich  in  soda,  in  which  nepheline  is  common,  are  the 
ones  that  contain  acmite  and  aegirine,  the  soda-pyroxenes. 

The  monoclinic  amphiboles  are  closely  parallel  in  their  occu- 
rence  and  relations  to  the  pyroxenes.  Tremolite  is  met  in  crystal- 
line limestones.  Actinolite  may  form  schistose  rocks  by  itself,  but 
much  the  most  important  variety  is  hornblende,  the  aluminous 
variety  corresponding  to  augite.  The  soda  amphibole  arfvedso- 
nite  is  rare. 

The  pyroxenes  and  amphiboles  are  often  collectively  referred  to 
as  the  bisilicates,  the  oxygen  of  the  base  being  to  the  oxygen  of 
the  silicon,  as  shown  in  the  first  two  formulas,  in  the  ratio  of  1:2. 
It  is  also  interesting  to  note  that  many  blast  furnace  slags  are  cal- 
culated on  the  basis  of  the.  formulas  for  pyroxene. 

THE  MICAS.  The  commonest  of  these  is  biotite  and  its  distribu- 
tion is  very  wide.  It  is  a  complex  silicate  involving  magnesia  in 
large  amounts  and  is  often  called  magnesia  mica  for  this  reason. 
The  other  bases  are  hydrogen,  potassium,  iron  and  aluminum,  and 
the  general  formula  is  (H,  K)2  (Mg,  Fe)2  Al2Si3O12.  It  is  impor- 
tant in  its  bearings  on  the  classification  of  rocks.  Phlogopite  is 
of  related  composition  but  is  almost  entirely  limited  to  crystalline 
limestones.  Muscovite,  from  its  richness  in  potash,  is  often  called 
potash  mica.  It  is  widespread  in  granites  and  schi  ts  and  as  an 
alteration  product.  Its  general  formula  is  (H,  K)  Al  SiO4. 

OLIVINE,  the  unisilicate   of  magnesium   and   iron,   2(Mg,Fe)O 
SiO2,  completes  the  list  of  silicates  which  are  of  the  first  order  of 
importance  in  igneous  rocks.     The  above  name  is  usually  employed 
in  preference  to  chrysolite.     Olivine  is  practically  limited  to  basic 
igneous  rocks. 

Zircon  and  titanite  are  interesting  microscopic  accessories,  but  as 
rock-making  minerals  they  arej..seldom  visible  to  the  naked  eye. 

Along  the  contacts  of  intrusions  of  heated  igneous  rocks,  and  in 
regions  where  the  original  sediments  have  undergone  strong  dy- 
namic disturbances,  with  oftentimes  attendant  circulations  of  waters 
more  or  less  heated,  a  series  of  characteristic  silicates  is  in  each 
case  developed.  Garnet,  tourmaline,  topaz,  andalusite,  scapolite 


INTRODUCTORY.  9 

and  biotite  are  especially  characteristic  of  the  former;  garnet, 
cyanite,  sillimanite,  staurolite,  biotite,  and  muscovite  of  the  latter. 
Epidote  results  when  feldspars  and  the  ferro-magnesian  silicates 
undergo  decay  and  alteration  in  proximity,  so  that  the  solutions 
afforded  may  react  on  one  another. 

The  hydrated  silicates  of  chief  importance  include  a  magnesian 
series,  embracing  talc  and  serpentine,  which  result  from  the  ferro- 
magnesian  minerals;  a  ferruginous  aluminous  series,  with  much  iron 
oxide,  usually  collectively  called  "  chlorite,"  and  derived  from  the 
iron-alumina  silicates ;  and  finally  kaolin,  the  hydrated  silicate  of  alu- 
mina that  is  chiefly  yielded  by  feldspar.  Zeolitic  minerals  are  also 
often  met,  but  rather  as  vein  fillings  and  in  amygdaloidal  cavities 
than  as  important  rock  makers. 

The  oxides  include  quartz  and  its  related  minerals  chalcedony 
and  opal,  and  the  oxides  of  iron — magnetite  and  hematite  and  the 
hydrated  oxide,  limonite.  With  these  should  be  mentioned  chro- 
mite  and  ilmenite  (menaccanit.e),  which  are  of  minor  importance. 
Quartz  is  found  in  all  rocks  high  in  silica.  Magnetite  and  hema- 
tite are  at  times  almost  abundant  enough  to  constitute  rocks 
themselves.  They  favor  igneous  and  metamorphic  varieties  when 
present  in  a  subordinate  capacity.  Magnetite  is  the  most  wide- 
spread of  all  the  rock-making  minerals.  Limonite  is  an  altera- 
tion product.  Chromite  is  practically  limited  to  the  basic  igneous 
rocks  and  their  serpentinous  derivatives.  Ilmenite  is  a  common 
accessory  in  many  igneous  rocks. 

The  cajbonates  are  calcite,  dolomite  and  siderite,  all  three  being 
really  members  of  an  unbroken  series  from  pure  carbonate  of  cal- 
cium, through  admixtures  of  magnesium  carbonate  to  pure  magne- 
site  on  the  one  hand,  or  with  increasing  carbonate  of  iron  to  pure 
siderite  on  the  other.  The  sulphates  of  moment  are  antiydrite  and  ?- 
gypsum,  the  latter  the  hydrous,  the  former  the  anhydrous  salt 
of  lime.  The  one  chloride  is  the  sodium  chloride,  rock  salt  or 
halite,  and  the  one  phosphate  is  apatite,  the  phosphate  and  chloride 
of  lime.  The  two  sulphides  of  iron,  pyrite  and  pyrrhotite  are  the 
only  ones  sufficiently  widespread  to  deserve  mention,  and  graphite 
is  the  chief  representative  of  the  elementary  substances,  although 
native  sulphur  might  perhaps  with  propriety  be  also  mentioned. 

We  speak  of  minerals  as  essential  and  accessory,  meaning  by 
the  former  term  those  that  constitute  a  large  part  of  the  rock,  and  that 
must  be  mentioned  in  the  definition  ;  by  the  latter  those  that  are 


io  A  HANDBOOK  OF  ROCKS. 

present  in  small  amounts  or  that  are  more  or  less  fortuitous. 
Primary  minerals  are  those  that  date  back  to  the  origin  of  the  rock, 
as  for  instance  the  ones  that  crystallize  out  from  a  molten  magma 
as  it  solidifies ;  secondary  minerals  are  formed  by  the  alteration  of  the 
primary.  Feldspars,  pyroxene  and  hornblende  are  good  illustra- 
tions of  the  former  ;  hydrated  silicates  of  the  latter. 

THE  PRINCIPLES  UNDERLYING  THE  CLASSIFICATION  OF  ROCKS. 

Rocks  must  of  necessity  be  classified  in  order  to  place  them  in 
their  natural  relations  so  far  as  possible  and  to  allow  of  their  syste- 
matic study.  At  the  same  time  they  are  so  diverse  in  their  nature 
and  origin  that  the  subject  is  not  an  easy  one.  They  must  however 
be  grouped  on  the  basis  of  their  structures  and  textures ;  or  of 
their  mineralogical  composition  ;  or  of  their  chemical  composition  ; 
or  of  their  geological  age ;  or  of  their  method  of  genes^  One  or 
several  of  these  principles  enter  into  all  schemes.  On  the  basis  of 
the  first,  rocks  have  been  classified  as  massive  and  stratified ;  as 
crystalline  and  fragmental  or  clastic,  each  with  subdivisions  on  one 
or  more  of  the  other  principles.  On  the  basis  of  the  second  we 
have  had  those  with  only  one  mineral  (simple  rocks)  and  those 
with  several  (complex  rocks).  The  chemical  composition  as  shown 
by  a  total  analysis  (bausch-analysis)  without  regard  to  special  min- 
eral components  is  of  almost  universal  application  in  a  subordinate 
capacity.  It  must  be  regarded  in  the  group  of  igneous  rocks  and 
in  those  that  are  deposited  from  solution,  chiefly  highly  calcareous 
or  highly  siliceous  rocks.  The  principle  of  geological  age  was 
formerly  much  valued  in  connection  with  igneous  rocks,  but  it  is  a 
thoroughly  exploded  one.  The  principle  of  origin  or  genesis  is 
the  most  philosophical  of  all  as  a  fundamental  basis,  but  while  in 
the  greater  number  of  cases  it  may  be  readily  applied  there  are 
some  puzzling  members  whose  entire  geological  history  is  not  well 
understood.  Very  early  in  the  development  of  the  subject  it 
was  appreciated  that  there  were  two  great,  sharply  contrasted 
groups,  according  as  they  had  consolidated  and  crystallized  from  a 
molten  condition  or  had  been  deposited  in  water  either  as  mechan- 
ical fragments  or  as  chemical  precipitates.  Widened  observa- 
tion, especially  in  arid  and  sandy  regions,  has  added  to  these  a  less 
important  group  of  those  whose  particles  have  been  heaped  to- 
gether by  the  wind.  They  are  called  the  eolian  rocks  and  will  be 
taken  up  together  with  the  aqueous,  with  which  they  have  many 


INTRODUCTORY.  n 

points  in  common.  Two  grand  divisions  have  therefore  been  es- 
tablished, the  igneous,  on  the  one  hand,  and  the  aqueous  and 
eolian  on  the  other. 

Even  a  limited  field  experience  soon  convinces  the  observer  that 
many  rocks  are  encountered  which  cannot  be  readily  placed  with 
either  of  the  two  great  classes  whose  origin  is  comparatively 
simple.  Rocks  for  instance  are  met  having  the  minerals  common 
to  the  igneous,  but  with  structures  that  resemble  those  of  sediments 
in  water. 

Great  geological  disturbances,  especially  if  of  the  nature  of  a 
shearing  stress,  may  so  crush  the  minerals  of  any  igneous  rock  and 
stretch  them  out  in  bands  and  layers  as  to  closely  imitate  a  re- 
crystallized  sediment.  The  baking  action  of  igneous  intrusions  on 
fine  sediments,  such  as  clays  and  muds,  makes  it  difficult  for  an 
observer,  without  the  aid  of  thin  sections  and  a  microscope  to  say 
where  the  former  sediment  ends  and  the  chilled  magma  begins. 
Sediments  buried  at  great  depths  and  subjected  to  heat  and  hot 
water  become  recrystallized  with  their  chemical  elements  in  new 
combinations.  These  excessively  altered  rocks  have  been  often 
grouped  into  a  separate,  so-called  "  metamorphic  "  division,  which 
was  a  sort  of"  omnibus"  of  unsolved  geological  problems.  This 
metamorphic  group  is  useful,  and  the  term  is  a  common  one  in  the 
science,  but  wherever  possible  it  is  well  to  appreciate  the  true 
affinities  of  its  members  which  though  altered  are  still  referable  to 
their  originals. 

In  the  following  pages  these  three  divisions  will  be  adopted,  but 
the  metamorphic  group  will  be  reduced  to  a  minimum  by  remark- 
ing, in  connection  with  descriptions  of  the  unaltered,  changes  that 
igneous  and  aqueous  undergo. 

We  take  up,  therefore,  in  this  order : 

A.  The  Igneous  Rocks. 

B.  The  Aqueous  and  Eolian  Rocks. 

C.  The  Metamorphic  Rocks, 


CHAPTER  II. 

GENERAL  INTRODUCTION  TO  THE  IGNEOUS  ROCKS.     CLASSIFICATION. 

The  Igneous  rocks  are  first  treated  because  they  have  been  the 
originals,  according  to  our  best  light,  from  which  all  the  others 
have  been  directly  or  indirectly  derived,  for  either  from  the  frag- 
ments, as  afforded  by  their  decay,  or  from  the  mineral  solutions, 
yielded  by  their  alteration,  possibly  in  the  primitive  history  of  the 
globe,  all  the  others  have  been  produced. 

The  igneous  rocks  occur  in  dikes,  sheets,  laccolites,  bosses 
and  yast  irregular  bodies,  for  which  we  have  no  single  term. 
Dikes  (spelled  also  dykes)  have  penetrated  fissures  in  other  rocks, 
and  have  solidified  in  them.  They  therefore  constitute  elongated 
and  relatively  narrow  bodies,  of  all  sizes,  from  a  fraction  of  an 
inch  in  thickness  and  a  few  feet  in  length,  to  others  a  thousand  or 
more  feet  across  and  miles  in  length.  Sheets  are  bodies  of  rela- 
tively great  lateral  or  horizontal  extent,  compared  with  their  thick- 
ness. They  are  either  surface  flows,  which  maybe  afterwards  bur- 
ied or  else  are  intruded  between  other  strata.  In  the  last  case 
they  are  often  called  laccolites,  especially  if  lenticular  in  shape. 
Roughly  tubular  masses,  such  as  might  chill  in  the  conduit  of  a 
volcano  are  called  necks.  Irregular,  projecting,  rounded  bodies 
are  called  bosses^.  The  enormous  masses  of  crystalline  rocks  like 
granite  that  often  cover  hundreds  of  square  miles,  and  that 
frequently  appear  to  have  fused  their  way  upward  by  melt- 
ing into  their  substance,  overlying  rocks,  are  called  bathy- 
lites.  They  have  in  most,  if  not  all  instances,  only  been  uncov- 
ered by  erosion,  for  the  name  means  a  rock  belonging  to  the 
depths  of  the  earth.  It  will  be  later  brought  out  that  the  character 
of  the  occurrence,  whether  as  dike,  surface  flow,  intruded  sheet, 
or  bathylite,  has  an  important  influence  on  the  texture. 

12 


IGNEOUS  ROCKS.     CLASSIFICATION.  13 

Igneous  rocks  are  characteristically  massive,  as  contrasted  with 
the  stratified  structure  of  the  sedimentary,  and  the  term  massive 
is  sometimes  employed  as  a  synonym  of  igneous.  Other  synony- 
mous terms  are  eruptive  and  anogene,  both  meaning  that  the  rocks 
have  come  up  from  below.  Many  years  ago  the  distinction  was 
made  between  those  that  have  crystallized  deep  within  the  earth, 
theplutonic  and  those  that  have  been  poured  out  on  the  surface, 
the  volcanic.  The  words  intrusive  and  effusive  or  extrusive  have 
been  employed  in  much  the  same  way.  Between  surface  flows 
and  deep-seated  masses  (bathylites)  and  their  characteristic  tex- 
tures, every  gradation  is  to  be  expected  and  is  met,  and  an  inter- 
mediate group  has  even  been  established  by  some  writers  for  rocks 
that  have  cooled  as  intruded  sheets  and  dikes.  This  three-fold 
distinction  is  not  carried  out  here,  the  two  extremes  being  believed 
to  illustrate  the  varieties  satisfactorily  when  accompanied  by  aux- 
iliary remarks  on  the  intermediate  types. 

We  are  tending  more  and  more  to  employ  the  word  structure  to 
describe  the  larger  features  of  a  rock,  as  for  instance  a  massive 
structure  as  against  a  stratified,  while  the  smaller  features  are  de- 
scribed as  textures,  as  for  instance  a  glassy  texture,  a  porphyritic 
or  a  granitoid,  terms  that  refer  to  characters  which  may  be  seen 
even  on  a  small  fragment.  Glassy  texture,  as  the  name  implies,  is 
that  of  glass  or  slag  and  has  no  definite  minerals.  It  results  when 
a  molten  magma  is  so  quickly  chilled  that  the  minerals  have  no 
opportunity  to  form.  Porphyritic  implies  larger  crystals,  well 
formed  or  corroded  and  rounded,  embedded  in  a  more  finely  crys- 
talline, or  even  in  a  glassy  "  ground-mass."  There  may  be  several 
sizes  and  kinds  of  these  crystals,  and  because  of  their  prominence 
in  the  rock  they  are  called  phenocrysts,  i.  e.,  apparent  crystals, 
but  phanerocryst  is  better  etymologically.  If  a  magma  crystal- 
lizes as  a  mass  of  very  fine  or  microscopic  crystals  without  pheno- 
crysts, its  texture  is  described  as  felsitic.  A  granitoid  or  granular 
texture  has  the  component  crystals  all  about  the  same  size,  and 
very  seldom  possessing  their  own  crystal  boundaries.  Strictly 
speaking,  there  is  no  groundmass  in  granitoid  rocks.  Sometimes 
from  a  local  abundance  of  mineralizers  (as  later  explained),  grani- 
toid rocks  have  small  cavities  into  which  the  component  minerals 
project  with  well  bounded  crystals.  Such  are  called  miarolitic. 

Textures  in  igneous  rocks  are  due  to  several  factors  that  have  in- 
fluenced the  development  of  the  magma  during  its  consolidation. 


I4  A  HANDBOOK  OF  ROCKS. 

The  most  important  are  chemical  composition,  temperature,  rate 
of  cooling,  pressure  and  the  original  presence  of  dissolved  vapors 
called  mineralizers.  The  fusibility  varies  with  the  chemical  com- 
position. The  most  acid  or  siliceous  magmas,  i.  e.  those  with  65- 
75%  SiO2  are  least  fusible.  When  molten  they  are  viscid  and  ropy. 
The  fusibility  increases  with  the  decrease  of  silica  down  to  the 
basic  rocks  with  40  to  50%  SiO2.  The  ultra-basic  rocks  which 
graduate  into  practically  pure  bases,  as  in  some  rare,  igneous  iron 
ores  are  less  fusible.  This  statement  that  acid  rocks  are  least  fusi- 
>ble  often  puzzles  a  student  who  is  familiar  with  blast  furnace  practice 
'and  the  composition  of  slags,  in  which  the  most  siliceous  are  re- 
garded as  most  fusible,  but  slags  themselves  as  a  comparison  of 
analyses  will  readily  show  are  to  be  paralleled  with  basic  rocks. 
The  importance  of  the  fusibility  as  regards  textures  lies  in  the 
fact  that  the  highly  siliceous  quickly  chill,  become  ropy  and 
freeze.  They  therefore  especially  yield  glasses.  The  easily  fusi- 
ble remain  fluid  to  lower  temperatures,  crystallize  out  as  min- 
erals to  a  greater  degree  and  seldom  yield  glasses.  They  flow 
farther  from  the  vent  and  tend  to  develop  the  porphyritic  or 
even  a  variety  of  granular  texture.  The  influence  of  temperature 
has  been  partly  outlined  in  speaking  of  composition,  but  it  will 
readily  appear  that  in  its  progress  to  the  surface  a  basic  magma 
might  stand  for  a  considerable  period  at  a  temperature  of  flu- 
idity, whereas  an  acid  magma  in  the  same  situation  would  con- 
solidate. The  rate  of  cooling  is  important.  Cooling  magmas  tend 
to  break  up  into  minerals.  As  a  general  thing  it  requires  a  very 
quick  chill  to  prevent  their  formation.  Hence  it  is  that  even  vol- 
canic glasses  which  appear  to  be  perfect  glass  to  the  eye  are  shown 
to  be  full  of  dusty,  microscopic  minerals  under  the  microscope. 
Volcanic  glasses  are  chiefly  found  on  the  outer  portions  of  flows  or 
dikes,  but  instances  are  known  where  sheets  of  them  are  very  thick, 
as  at  Obsidian  Cliff  in  the  Yellowstone  Park.  The  common  experi- 
ence with  lavas  is  that  certain  crystals  develop  to  notable  size,  it 
may  be  an  inch  or  more  in  diameter,  while  the  magma  stands  be- 
neath the  surface,  in  circumstances  favorable-  to  their  formation. 
These  are  then  caught  up  in  the  moving  stream  and  brought  to  the 
surface  or  near  it  where  the  final  consolidation  takes  place  and  fixes 
them  in  the  so-called  ground  mass.  A  quick  chill  makes  a  fine- 
grained groundmass  when  not  a  glassy  one,  a  slow  cooling  yields 
one  more  coarsely  crystalline,  but  in  the  final  cooling,  or  consoli- 

\ 


IGNEOUS  ROCKS.     CLASSIFICATION.  15 

dation  at  or  near  the  surface,  crystals  are  seldom  if  ever  developed 
of  a  size  commensurable  with  those  formed  in  the  depths.  By  this 
process  of  partial  crystallization  below  and  final  consolidation  on 
the  surface,  the  porphyritic  texture  is  almost  always  developed, 
but  in  strict  accuracy  it  should  be  stated  that  cases  are  known 
where  phenocrysts  appear  to  have  formed  in  lavas  after  coming  to 
rest.  Magmas  also  flow  to  the  surface  with  no  phenocrysts  (or 
"  intratelluric  "  crystallizations)  and  then  consolidate  not  as  glass, 
but  as  finely  crystalline  aggregates,  practically  all  groundmass. 
The  resulting  texture  is  called  felsitic. 

Pressure,  such  as  is  developed  upon  a  magma  deep  within  the 
earth  or  during  its  passage  to  the  surface  is  thought  to  exert  an 
influence  upon  the  formation  of  many  phenocrysts  and  to  be  neces- 
sary for  their  development.  Dissolved  vapors,  such  as  steam, 
hydrofluoric  and  boracic  acids  are  also  important  factors.  Acidic 
magmas  are  more  generally  provided  with  them  than  basic,  and 
where  locally  abundant  they  lead  to  variations  both  in  the  mineral 
composition  and  texture  at  different  places  in  the  consolidated 
rock.  They  may  prevent  the  development  of  glass,  and  cause  a 
sheet  such  as  Obsidian  Cliff,  in  the  Yellowstone  Park,  to  present 
alteijpions  of  glassy  and  stony  layers,  the  latter  being  formed  of 
microscopic  crystals. 

A  word  should  be  added  about  the  chemical  composition  of 
rocks  and  about  the  interpretation  of  analyses  before  the  rocks 
themselves  are  taken  up.  The  analyses  are  reported  in  percent- 
ages of  oxides,  for  the  most  part,  and  these  are  arranged  in  the  fol- 
lowing series,  SiO2,  A12O3,  Fe2O3,  FeO,  CaO,  MgO,  Na2O,  K2O, 
H2O.  In  order  to  have  anhydrous  materials,  it  is  customary  to 
ignite  and  determine  loss  on  ignition.  This  loss  includes  both 
H2O  and  CO2  and  where  large  throws  uncertainty  over  the  relations 
of  the  elements  left  behind,  because  of  the  evident  advance  of  decay. 
Small  percentages  of  other  oxides  are  quite  invariably  present 
and  in  refined  work  are  determined.  These  are  TiO2,  MnO,  NiO, 
BaO,  SrO,  S,C1,  P2O5,  Li2O,  and  even  rarer  ones.  They  are  how- 
ever always  in  very  small  quantity.  We  often  recast  an  anal- 
ysis, by  dividing,  as  in  the  determination  of  a  mineralogical 
formula,  each  percentage  by  the  molecular  weight.  We  thus  get 
numerical  molecular  ratios  which  indicate  the  relative  numbers  of 
individual  molecules  and  enable  us  to  draw  conclusions  as  to  the 
way  they  are  combined  with  one  another  in  the  component 


1 6  A  HANDBOOK  OF  ROCKS. 

minerals.  Variations  in  chemical  composition  entail  variations  in 
resulting  minerals,  but  it  is  also  true  that  the  same  magma,  if 
consolidating  under  different  physical  conditions  of  heat,  pressure, 
etc.,  at  different  times  may  yield  somewhat  different  minerals,  for 
instance,  hornblende  instead  of  augite,  or  vice  versa.  A  study  of 
analyses  soon  makes  one  more  or  less  familiar  with  the  minerals 
that  would  necessarily  result.  The  more  important  points  are  the 
amounts  of  silica,  of  the  alkalies  and  alkaline  earths,  of  iron  oxides 
and  of  alumina.  For  instance,  as  a  rule  only  magmas  high  in 
SiO2,  yield  quartz,  for  otherwise  it  would  combine  with  the  bases. 
Much  K2O  is  necessary  for  an  orthoclase  or  leucite  rock,  but  much 
Na2O  for  one  with  nepheline.  MgO  in  relatively  large  amount  is 
required  to  yield  olivine  or  an  orthorhombic  pyroxene,  and  when 
feldspars  drop  away  and  rocks  become  very  basic  we  expect 
high  CaO,  MgO,  FeO,  Fe2O3,  and  low  SiO2.  In  rocks  tested  as 
building  stone,  the  significance  of  sulphur  is  important  and  very 
little  should  be  present,  for  it  indicates  pyrites.  It  should  never 
reach  I  %. 

The  specific  gravity  or  density  of  a  rock  is  an  important  feature 
in  its  practical  bearings.  While  it  may  in  ice  be  less  than  I ,  and 
in  coals  and  certain  carbonaceous  deposits  drop  as  low  as  1.25,  and 
in  very  porous  sandstones  reach  2. 25,  yet  in  the  common  rocks  it 
is  seldom  below  2.50,  and  ranges  from  this  to  over  3.00.  Granites 
are  usually  about  2.65,  but  basic  rocks,  rich  in  iron,  attain  to  the 
higher  limits,  even  above  3.0.  Determinations  are  important  in 
those  rocks  used  for  building  purposes,  and  are  expressed  in 
pounds  per  cubic  foot. 

Of  recent  years  we  have  come  to  regard  molten  magmas  as  es- 
sentially solutions  of  some  compounds  in  others,  and  to  appreciate 
that  solutions  do  not  cease  to  be  such,  even  when  the  temperature 
is  very  high.  It  results  from  this  that  the  crystallization  of  the 
minerals  of  an  igneous  rock  takes  place  from  the  magma  as  this 
in  its  cooling  successively  reaches  a  point  of  saturation  for  the  salt 
in  question.  The  least  soluble  thus  separate  the  earliest  of  all, 
and  then  the  others  in  order  ;  but  as  the  pressure  under  which 
they  rest  is  also  a  factor,  and  this  is  subject  to  variation,  as  indeed 
is  the  temperature  during  movement  to  the  surface,  one  mineral's 
period  of  formation  may  overlap  another's  more  or  less.  The 
order  of  formation  will  be  determined  by  the  laws  of  thermo- 
dynamics and  necessarily  the  mineral  that  develops  the  most 


IGNEOUS  ROCKS.     CLASSIFICATION.  17 

heat  in  crystallizing  will  be  the  first  to  develop.  As  a  general 
rule,  the  relations  of  the  minerals  in  rocks  show  that  the  earliest 
to  form  are  apatite;  the  metallic  oxides  (magnetite,  ilmenite, 
hematite);  the  sulphides  (pyrite,  pyrrhotite);  zircon  and  titanite. 
These  are  often  called  the  group  of  the  ores.  Next  come  the 
ferromagnesian  silicates,  olivine,  biotite,  the  pyroxenes  and. 
hornblende.  Next  follow  the  feldspars  and  feldspathoids,  nephe- 
line  and  leucite,  but  their  period  often  laps  well  back  into  that  of 
the  ferromagnesian  group.  Last  of  all,  if  any  excess  of  SiO2  re- 
mains, it  yields  quartz.  In  the  variation  of  the  conditions  of 
pressure  and  temperature  just  referred  to,  it  may  and  does  often 
happen  that  crystals  are  again  redissolved  in  the  magma,  or  are  re- 
sorbed,  as  it  is  called ;  and  it  may  also  happen  that  after  one  series 
of  minerals,  anusually  of  large  size  and  of  intratelluric  origin,4w*¥e/ 
formed.the  series  is  again  repeated  on  a  small  scale  as  far  back  as 
the  ferromagnesian  silicates.  Minerals  of  a  so-called  second  gen- 
eration thus  result,  but  they  are  always  much  smaller  than  the 
phenocrysts,  and  are  characteristic  of  the  groundmass. 

It  results  from  what  has  been  stated  that  the  residual  magma  is 
increasingly  siliceous  up  to  the  final  consolidation,  for  the  earliest 
crystallizations  are  largely  pure  oxides.  It  is  also  a  striking  fact 
that  the  least  fusible  minerals,  the  feldspars  and  quartz,  are  the 
last  to  crystallize. 

In  the  matter  of  the  study  and  determination  of  a  rock  species, 
especially  of  an  igneous  rock,  it  is  desirable  to  procure  materials  as 
fresh  and  unaltered  as  possible.  If  feldspars  have  all  changed  to 
kaolin  and  clay,  and  if  ferromagnesian  silicates  are  merely  chlorite 
or  serpentine,  and  if  secondary  quartz,  calcite  and  the  like  have 
formed,  it  is  very  difficult  if  not  impossible  to  draw  correct  or  even 
well-grounded  inferences.  Rocks  near  ore  bodies  are  very  often 
of  this  character. 

Bearing  in  mind  these  differences  of  texture  and  the  causes  of 
them,  it  is  possible  to  group  igneous  rocks  in  such  arrangement  that 
they  can  be  intelligently  studied,  and  identified  with  a  reasonably 
close  approximation  to  the  truth.  It  should  be  appreciated, 
however,  that  with  finely  crystalline  rocks,  whose  components  are 
too  small  for  the  unassisted  eye,  the  microscope  is  the  only  re- 
source, and  with  this  as  an  aid  much  greater  subdivision  can  be 
attained.  The  object  here  in  view  is  to  limit  the  discussion 
purely  to  the  study  without  the  microscope. 


i8                         A    HANDBOOK  OF  ROCKS. 

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1 

IGNEOUS  ROCKS.     CLASSIFICATION.  19 

The  scheme  of  classification  of  the  igneous  rocks  has  three 
principles  underlying  it,  viz  :  texture,  mineralogical  composition  and 
chemical  composition.  The  textures  are  live,  glassy,  felsitic,  por- 
phyritic,  fragmental  and  granitoid,  and  the  table  is  arranged  from 
top  to  bottom  so  that  they  come  in  this  order.  The  arrangement^ 
is  adopted  because  it  brings  the  glassy  which  are  the  simplest  of 
all  rocks  at  the  outset,  where  they  can  be  best  taken  up  by  the 
beginner.  The  rocks  are  arranged  from  left  to  right  on  a  mineral- 
ogical principle,  and  chiefly  on  the  basis  of  the  predominant  feld- 
spar present,  as  is  the  usual  custom.  This  also  makes  possible  a 
general  succession  from  those  most  acidic  on  the  left  to  those  most 
basic  on  the  right,  but  while  this  is  true  for  the  extremes  it  is  not 
strictly  so  for  intermediate  points  because  dacites  and  quartz- 
diorites  are  far  higher  in  silica  than  are  phonolites  and  nepheline- 
syenites,  and  even  than  trachytes  and  syenites.  The  general  range 
of  silica  is  indicated  on  the  lowest  line.  At  the  same  time  the  im- 
portance of  the  bases  is  not  to  be  overlooked  arid  subsequent 
tables  of  analyses  are  given  so  as  to  show  the  range. 

The  general  and  larger  truths  of  igneous  rocks  are  fairly  well 
brought  out  in  condensed  tables  of  this  character,  although  excep- 
tional cases  are  known  that  would  require  its  modification.  But  no 
attempt  has  been  made  to  confuse  the  larger  truths  by  mention  of 
the  rarer  occurrences,  for,  as  before  stated,  only  ordinary  examina- 
tion is  assumed  in  connection  with  this  text.  When  rare  and  ex- 
'ceptional  varieties  are  met  they  should  be  placed  in  the  hands  of  a 
microscopical  worker.  It  should  also  be  appreciated  in  connection 
with  the  table  that  groups  of  rocks  shade  into  one  another  by  im- 
perceptible gradations  and  that  .they  are  not  marked  ofF  with  the 
sharpness  of  ruled  spaces.  In  general  the  glassy,  felsitic  and  por- 
phyritic  constitute  the  lavas  or  surface  flows,  the  dikes  and  the 
laccolites,  while  the  granitoid  rocks  are  the  deep-seated,  or  abyssal 
ones,  but  there  are  cases  where  the  latter  show  porphyritic  tenden- 
cies and  others  wherein  the  former  shade  into  granitoid  textures. 


CHAPTER   III. 

THE  IGNEOUS  ROCKS,  CONTINUED.      THE  GLASSES.      THE  ROCKS 

WHOSE  CHIEF  FELDSPAR  is  ORTHOCLASE.     THE  PHONO- 

LITES  AND  NEPHELINE-SYENITES. 

THE  GLASSES. 


SiO2 

A1203 

Fe203 

FeO. 

CaO. 

MgO. 

K20 

Na20 

Loss. 

Sp.  Gr 

I. 

79-49 

ii.  60 

0-33 

0.49 

1.64 

0.09 

1.52 

4.04 

0.68 

2. 

76.20 

J3-I7 

0-34 

0-73 

0.42 

0.19 

4.46. 

4.31 

0-33 

2-352 

3- 

75-52 

14.11 

1.74 

0.08 

0.78 

O.IO 

3-63 

3-92 

0-39 

2.342 

4- 

74.70 

13.72 

I.OI 

0.62 

0.78 

0.14 

4.02 

3-90 

0.62 

2-345 

5- 

74-05 

13.85 

tr. 

.    . 

090 

0.07 

4-31 

4.60 

2.20 

6. 

74.05 

12.97 

2-73 

.    . 

0.12 

0.28 

5-11 

3.88 

O.22 

2-37 

7- 

74.oi 

12.95 

.    . 

1.42 

0-99 

0.48 

4-65 

5-34 

0.29 

2.391 

8. 

72.87 

12.05 

'•75 

.    . 

I.30 

1.  10 

tr. 

6.13 

3-0° 

9- 

71.6 

12.0 

I.O 

,    , 

I.I 

0.2 

4-3 

2-5 

7-4 

10. 

7!-56 

13.10 

0.66 

0.28 

0.74 

0.14 

4.06 

3-77 

5-52 

ii. 

65-13 

I5-73 

2.24 

1.86 

3.62 

1.42 

3-96 

2-93 

2-43 

12. 

60.5 

19.1 

4.2 

o-3 

0.6 

0.2 

3-5 

10.6 

.    . 

2.48 

13- 

54.28 

14.83 

J4-73 

.    . 

7.02 

3-65 

1.27 

4.22 

.    . 

2.704 

14. 

50.82 

9.14 

7-33 

7-03 

11.63 

7.22 

1.02 

3.06 

1.74 

2.66 

i5- 

45-73 

20.15 

12.46' 

.   . 

8.67 

3-59 

4.II 

5-74 

0.12 

i.  Pumice,  Cinder  Cone,  Calif.,  J.  S.  Diller,  Bull.  79,  U.  S.  G.  S.,  p.  29.  2.  Black 
Obsidian,  TewanMtns.,  N.M.,  J.  P.  Iddings,  7th  Ann.  Rep.  U.  S.  G.  S.,  219.  3.  Red 
Obsidian,  Yellowstone  Park,  J.  P.  Iddings,  7th  Ann.  Rep.  U.  S.  G.  S.,  219,  also 
FeS2  o.i  I.  4.  Black  Obsidian,  Yellowstone  Park,  J.  P.  Iddings,  7th  Ann.  Rep.  U.  S. 
G.  S.  219,  also  FeS2  0.40.  5.  Scoriaceous  Obsidian,  Mono  Lake,  Cal.,  I.  C.  Russell,  8th 
Ann.  Rep.  U.  S.  G.  S.,  380.  6.  Obsidian,  Lipari  Is.  Abich.  Vulk.  Ersch.  62.  7.  Obsidian 
from  Andesite,  Clear  Lake,  Cal.,  G.  F.  Becker,  Mon.  XIII.,  U.  S.  G.  S.  104.  8.  Perlite, 
Hungary,  Kalkowsky,  Elemente  der  Lith.  p.  75.  9.  Pitchstone,  Meissen,  Lemberg,Z.  d.d. 
g.  G.  XXIX  508.  10.  Pitchstone,  Silver  Cliff,  Colo.,  W.  Cross,  Phil.  Soc.  Wash.  XL 
420.  ii.  Andesitic  perlite,  Eureka,  Nev.  Hague.  Mono.  XX.  U.  S.  G.  S.  264.  12. 
Phonolite  obsidian,  Teneriffe,  Abich,  Vulc.  Ersch.  62.  13.  Hyalomelane,  Ostheim, 
Germany,  Lemberg  Z.  d.  d.  g.  G.  XXXV.  570.  14.  Pele's  Hair,  Hawaii,  Cohen  N.  J. 
1880,  II.  41.  15.  Tachylyte,  Gethurms,  Germany,  Lemberg,  See  No.  13. 

Comments  on  the  analyses.  An  examination  of  the  table  of  analy- 
ses indicates  that  the  magmas  are  high  in  SiO2,and  relatively  low  in 

2O 


THE    VOLCANIC  GLASSES.  21 

•/»  / 

all  other  bases  except  the  alkalies.  The  high  Na2O  of  Number  12 
is  worthy  of  remark,  because  this  is  the  rule  with  a  nepheline  rock. 
The  percentages  under  the  column  headed  loss,  which  practically 
indicate  the  H2O  present  are  characteristic  for  different  varieties. 
They  are  low  in  the  case  of  obsidians,  Nos.  2,  3,  4,  6,  7 ;  un- 
usually high  in  No.  5,  described  by  Russell  as  scoriaceous  obsidian; 
still  higher  in  the  perlites  Nos.  8,  1 1  ;  and  reach  a  maximum  in 
the  pitchstones  Nos.  9  and  10. 

Basic  glasses  are  seldom  sufficiently  free  from  included  crystals 
as  really  to  be  separable  from  the  porphyritic  rocks.  Frothy  and 
cellular  crusts,  do  however,  appear  on  lava  streams,  and  are  known 
as  scorias,  and  rare,  homogeneous  glasses  have  been  called  tachy- 
lyte  and  hyalomelane. 

Varieties.  The  chief  glasses  are  obsidian,  pumice,  perlite  and 
pitchstone.  The  name  obsidian,  is  applied  to  homogeneous 
glasses  with  low  percentages  of  water.  The  word  is  of  classic  and 
ancient  origin  and  is  now  used  with  a  prefixed  name  for  all  glasses, 
such  as  rhyolite-obsidian,  basalt-obsidian,  etc.  Pumice  is  an  ex- 
cessively cellular  glass,  caused  by  expanding  steam  bubbles. 
Perlite  is  a  glass  broken  into  small  onion-like,  individual  masses, 
by  concentric  cracks,  from  contractions  in  cooling.  The  concentiic, 
shelly  masses  lie  in  between  intersecting  series  of  larger,  straight 
cracks;  the  perlites  have  considerable  water,  usually  2-4%.  The 
word  is  also  written  pearlstone,  and  was  suggested  by  the  fancied 
resemblance  of  the  concentric  shells  to  the  familiar  gem.  Pitch- 
stone  is  a  homogeneous  glass,  like  obsidian,  but  contains  5~IO% 
of  water.  Pitchstones  have  often  a  more  resinous  appearance  than 
obsidians,  but  there  is  no  very  essential  difference  apparent  to  the 
eye.  The  name  was  formerly  used  for  glasses  of  earlier  geological 
age  than  the  obsidians.  Obsidians  are  usually  black  or  red,  with 
translucent  edges  ;  pitchstones  are  mostly  reds  and  greens,  but  thin 
slivers  are  practically  colorless ;  all  the  glasses  contain  dusty, 
embryonic  crystals,  gas  pores,  and  sometimes  skeleton  crystals  of 
larger  growth,  and  even  a  few  phenocrysts  which  are  often  ar- 
ranged in  flow  lines  and  swirling  eddies.  Almost  all  large  de- 
velopments of  the  glasses  show  dense,  stony  or  lithoidal  layers, 
and  streaks,  that  are  due  to  the  development  of  minute  crystals  of 
feldspar  and  quartz,  which  may  be  arranged  in  radiating  rosettes, 
called  spherulites.  The  individual  crystals  are  not  often  large 
enough  to  be  seen  with  the  unassisted  eye.  Expanded,  bubble- 


22  A   HANDBOOK  OF  ROCKS. 

like  cavities  are  also  met,  with  perhaps  several  concentric  walls, 
on  which  at  times  are  perched  little  well-formed  crystals.  These 
cavities  are  called  lithophysse,  i.  e.t  stone  bubbles.  Topaz,  quartz, 
tridymite,  feldspars,  fayalite  and  garnet  have  been  found  in  beauti- 
ful crystals  in  them.  The  lithophysae  are  due  to  the  influence 
and  escape  of  mineralizers,  and  may  reach  a  diameter  of  over  an 
inch. 

Relationships. — The  glasses  are  all  mere  varieties  of  volcanic  rocks, 
which  a  quick  chill  has  prevented  crystallizing.  At  the  same  time, 
it  is  only  possible  by  field  associations  or  by  chemical  analysis  to 
refer  them  to  their  corresponding  porphyritic  types,  although  in 
the  great  majority  of  cases  they  are  formed  from  rhyolitic  magmas. 

Geological  Occurrence. — The  glasses  sometimes  appear  as  inde- 
pendent sheets  and  dikes  ;  more  often  they  are  on  the  surface  of 
well  crystallized  lava-sheets  or  on  the  outer  portions  of  dikes. 

Alteration. — Glasses  resist  alteration  notably  well,  but  in  the  long 
run  are  subject  to  decay  along  cracks  and  exposed  surfaces.  They 
yield  quartz,  kaolin  and  fine,  scaly  muscovite.  In  instances  they 
devitrify,  as  it  is  called,  or  break  up  into  aggregates  of  quartz  and 
feldspar  in  excessively  minute  crystals,  so  that  we  can  only  trace 
them  back  to  the  original  glass,  by  the  flow  lines,  spherulites,  etc., 
that  still  remain.  Such  devitrified  forms  have  been  called  by  F. 
Bascom,  apobsidian.  Petrosilex  is  an  older  term  applied  to  these 
and  other  similar  rocks,  and  felsite  has  been  also  used. 

Distribution. — The  glasses  are  widespread  in  the  West.  Obsi- 
dian Cliff,  in  the  Yellowstone  Park,  yields  black,  red  and  stony 
varieties,  and  has  been  made  a  type  locality  by  the  studies  of  J.  P. 
Iddings.  Silver  Cliff,  Colorado,  has  furnished  some  remarkable 
pitchstones,  described  by  Whitman  Cross.  The  extinct  volcanoes 
of  New  Mexico,  Utah,  Montana  and  around  Mono  Lake,  California, 
are  well  known  localities.  Alaska  has  supplied  much  from  near 
Fort  Wrangel,and  in  Mexico  and  Iceland  are  other  prolific  sources. 
Along  the  Atlantic  Coast  there  are  only  the  devitrified  glasses  of 
ancient  (pre-Cambrian)  volcanoes.  These  are  well  developed  in 
New  Brunswick,  Maine,  Massachusetts  and  Pennsylvania.  Abroad 
the  obsidian  of  the  Lipari  Islands  is  a  famous  one,  and  the  perlites 
of  Hungary  supply  the  usual  type  specimens  in  our  collections. 
The  best  known  of  all  pitchstones  are  found  at  Meissen,  near  Dres- 
den, in  Saxony,  and  on  the  island  of  Arran,  off  the  west  coast  of 
Scotland. 


THE  RHYOLITES.  23 

THE  RHYOLITES. 


Si02 

A1203 

Fc20, 

FeO 

CaO 

MgO 

K20 

Na20 

Loss.  Sp.  Gr. 

I. 

83.59 

5-42 

tr. 

tr. 

3-44 

tr. 

i-37 

5-33 

0.76 

2-54 

2. 

78.95 

10.22 

3-2J 

.    . 

1.84 

0.14 

1.76 

4.18 

3- 

77-5 

9-7 

6.1 

. 

5-8 

°-3 

0.4 

4- 

75.20 

12.96 

o-37 

0.27 

0.29 

O.I2 

8.38 

2.O2 

0.58 

5- 

73-91 

i5-29 

.   . 

0.89 

0.77 

.     . 

4-79 

3-62 

1.19 

6. 

73-°7 

11.78 

2.30 

2.02 

o-39 

6.84 

I.I9 

2.24 

7- 

71.12 

14.58 

1.69 

.    . 

I.50 

0.15 

6.01 

3.26 

0-95 

8. 

70.92 

13.24 

3-54 

0.66 

1.42 

0.23 

4-25 

4-28 

o-57 

9- 

70.74 

14.68 

0.69 

0.58 

4.12 

0.28 

2-59 

2.29 

2.09 

2.68 

10. 

68.84 

15-73 

.    . 

3" 

3.58 

0.90 

3-59 

2.89 

1.50 

2.4 

n. 

68.10 

14-97 

2.78 

1.  10 

3-°4 

1.  10 

2-93 

3-46 

1.28 

2.636 

12. 

67.20 

H-95 

5-19 

.   . 

0.30 

2.39 

0.89 

4-OO 

2.13 

13- 

66.91 

14-13 

5.00 

.   . 

2-35 

0.95 

5-40 

3-86 

1.42 

14. 

66.60 

16.69 

2.06 

0-93 

1.40 

1.15 

5-23 

.2.46 

1.70 

243   . 

15- 

63-63 

17.42 

0.15 

5.76 

2.86 

.    . 

5-54 

4-52 

0.15 

I.  Soda-rhyolite,  Berkeley,  Cal.  Palache.  Bull.  Geol.  Dept.  Univ.  Calif.  I.  61.  2 
Rhyolite.  Iceland,  Backstrom,  Contrib.  to  Icelandic  Liparites.  3.  Rhyolite,  Wales, 
A.  Harker,  Bala  Vole.  Ser.  13.  4.  Rhyolite,  Silver  Cliff,  Colo.  Cross.  Colo.  Sci.  Soc. 
Dec.  5,  1887,  229.  5.  Rhyolite,  Pinto  Peak,  Eureka,  Nev.  A.  Hague,  Mono.  XX., 
264.  6.  Rhyolite,  McClelland  Peak,  Washoe  Dist,  Nev.  F.  A.  Gooch,  Bull.  17,  U. 
S.  G.  S.  33,  7.  Rhyolite,  Island  of  Ponza,  near  Naples.  Quoted  by  Kalkowsky, 
Elem.  d.  Lith.,  p.  75.  8.  Rhyolite,  Yellowstone  Park.  Idding's  Origin  Igneous 
Rocks,  Tab.  i.  9.  White  Porphyry,  Leadville,  Colo.,  Cross,  Mono.  XII.,  U.  S.  G.  S:  326. 
10.  Rhyolite,  Lassen's  Peak,  Cal.,  Fortieth  Paral.  Survey  I.  652.  II.  Gray  Porphyry, 
Leadville,  Colo.,  Mono.  XII.  U.  S.  G.  S.  332.  12.  Quartz  Porphyry,  Flagstaff  Hill,  Colo., 
Palmer  &  Fulton,  Colo.  Sci.  Soc.  III.,  356  13.  Rhyolite,  Hungary,  v.  Hauer,  Verh. 
d.  k.  k.  R.  1867,  118.  14.  Quartz  Porphyry,  Upper  Quinnesec  Falls,  Mich.,  G.  H. 
Williams,  Bull.  62,  U.  S.  G.  S.  120.  15.  Quartz  Porphyry,  Waterville,  N.  H.  Hawes, 
N.  H.  Geol.  Surv.  III.  178. 

Comments  on  the  Analyses. — The  analyses  illustrate  the  ranges 
of  the  various  molecules.  No.  I  is  a  very  exceptional  rock,  alike 
in  its  high  SiO2  and  CaO,  and  low  A12O3.  The  gradual  increase  of 
A12O3  in  all  the  others,  with  decrease  of  SiO2,  and  in  general  the 
same  relation  as  regards  CaO  are  worthy  of  remark,  as  is  the  pre- 
vailingly low  MgO.  Sometimes  K2O,  sometimes  Na2O,  is  in  ex- 
cess, and  this  brings  out  the  reason  why  we  spoke  of  orthoclase  as 
the  chief  feldspar,  not  as  the  only  one  in  the  table,  p.  19.  The 
specific  gravity  is  in  general  low. 

Varieties. — Rhyolites  proper  are  porphyritic  rocks  with  pheno- 
crysts  of  quartz,  alkali-feldspar,  usually  orthoclase,  of  biotite,  and  less 
commonly  hornblende  and  augite,  in  a  ground*mass  that  is  either 
glassy,  or  a  finely  crystalline  aggregate  of  quartz  and  feldspar,  or 


24  A   HANDBOOK  OF  ROCKS. 

both.  The  name  rhyolite  was  coined  from  the  Greek  verb  to  flow,  on 
account  of  the  frequent  flow  structure.  It  is  the  name  mostly  used 
\J  in  America  and  England,  whereas  liparite  (from  the  Lipari  Islands) 
and  quartz-trachyte  are  employed  in  Europe.  A  variety  with  very 
little  groundmass  and  an  approximation  to  a  granitoid  texture  is 
called  nevadite,  from  the  State  of  Nevada.  In  former  years  a  dis- 
tinction was  made  between  the  volcanic  rocks  of  pre-Tertiary  age 
and  those  of  later  date,  and  as  against  the  later  rhyolites  the  older 
were  called  quartz-porphyries,  but  the  distinction  has  no  serious 
foundations.  At  present  some  authors  define  quartz-porphyries  as 
rocks  corresponding  to  the  rhyolites,  but  which  have  crystallized  as 
intruded  sheets,  laccolites,  sills  and  dikes.  Nevertheless,  while  this 
distinction  has  some  force,  in  any  extensive  collection  of  specimens 
no  very  notable  difference  can  be  detected  in  the  hand  specimens 
even  by  a  very  practiced  observer.  Rhyolites  from  the  inner  parts 
of  thick  surface  flows,  and  from  their  branching  dikes,  can  be 
found,  especially  when  alteration  has  advanced  somewhat,  to  match 
all  quartz-porphyries,  but  as  a  general  rule  quartz-porphyries  are 
denser  and  less  cellular  than  rhyolites.  The  meaning  of  both 
words  should  be  well  understood  on  account  of  their  presence  in 
the  literature.  Certain  microscopic  structures  due  to  the  inter- 
penetration  of  quartz  and  feldspar  are  also  seen  in  the  quartz  por- 
phyries (i.  e.,  intruded  sheets),  that  are  seldom  met  in  surface 
flows,  but  these  have  no  value  in  ordinary  study.  An  old  and 
very  useful  term  is  felsite,  which  has  .been  applied  especially  to 
acidic  lavas  of  ancient  geological „  date,  that  lack  phenocrysts, 
wholly  or  largely.  Whether  they  correspond  to  rhyolites  or  to 
less  acidic  magmas,  such  a's. trachytes,  is  not  always  apparent  with- 
out chemical  analyses.  Felsites  are  dense,  usually  green,  red  or 
gray  rocks,  which  only  indicate  to  the  eye  that  they  are  very  finely 
crystalline.  They  really  consist  almost  entirely  of  minute  quartz 
and  feldspar  crystals,  practically  a  groundmass  without  pheno- 
crysts, but  it  is  not  always  apparent  whether  they  represent  origi- 
nal crystallizations  from  fusion,  or  devitrified  obsidians  (apob- 
sidians)  or  recrystallized  tuffs,  all  of  which  have  been  demonstrated 
in  one  place  or  another.  Those  certainly  derived  from  rhyolites 
have  been  called  aporhyolites. 

Rhyolites  high  in  soda  are  called  soda-rhyolites,  or  pantellerites, 
(see  analyses  I  and  2).  Ancient  rhyolites  (quartz-porphyries,  fel- 
sites)  rich  in  soda  have  been  called  quartz-keratophyr(see  analysis 


THE  RHYOLITES.  25 

12),  the  distribution  being  made  because  the  feldspar  present  must 
be  anorthoclase  as  against  true  orthoclase.  As  the  true  porphy- 
ritic  texture  graduates  into  the  granitoid,  we  have  intermediate 
rocks  called  granite  porphyries,  of  which  mention  is  made  under 
granites. 

Mineralogical  composition. — The  principal  minerals  present  of  re- 
cognizable size  are  quartz,  in  rounded  or  .doubly  terminated  (i.  e.y  di- 
hexagonal)  pyramids  with  practically  no  prism  faces  ;  and  feldspar, 
including  orthoclase,  less  often  anorthoclase,  and  a  soda-lime  plagio- 
clase  belonging  in  the  series  from  albite  to  oligoclase.  Biotite  is 
much  the  commonest  dark  silicate,  although  hornblende,  and  less 
often  augite,  are  occasional.  It  is  important  to  appreciate  that  the 
dark  silicates  are  vastly  inferior  in  quantity  to  the  light  ones.  As 
regards  the  groundmass,  by  the  unaided  eye,  we  can  only  deter- 
mine that  it  is  glassy  (called  also  hyaline),  or  finely  crystalline, 
that  is,  felsitic,  or  coarsely  crystalline.  Vesicular  groundmasses  are 
met  in  surface  flows,  not  in  intruded  masses.  Lithophysae  also 
occur  in  rhyolites  as  well  as  in  volcanic  glasses. 

Relationships. — Rhyolites  pass  by  insensible  gradations  into 
glasses  on  one  side,  trachytes  on  another,  granites  on  a  third  and 
dacites  on  a  fourth.  Without  the  microscope  rhyolites  can  only 
be  identified  with  certainty  by  recognizing  the  quartz,  and  may 
then  be  confused  with  dacites.  The  striated  feldspar  of  the  latter 
is  our  chief  means  of  distinction  between  the  two. 

Alteration. — Ordinary  decay  leads  to  the  formation  of  clays  and 
kaolin:  In  metamorphic  alterations  the  rhyolites  pass  into  very 
finely  crystalline  aggregates  of  quartz  and  feldspar,  and  then  it  is 
difficult  to  decide  what  minerals  are  original  and  what  secondary, 
and  whether  the  original  rock  was  a  massive  one  or  a  tuff.  Shear- 
ing stresses  develop  schistose  structures,  and  when  decay  is  further 
superadded  sericite  schists  may  result  that  are  extremely  difficult 
geological  problems. 

Distribution. — Rhyolites  are  common  in  the  Western  States, 
being  well  known  in  the  Black  Hills  ;  the  Yellowstone  Park ;  in 
Colorado,  where  Chalk  Mountain,  near  Leadville,  is  a  type  locality 
for '  nevadite ;  in  Nevada,  both  near  Eureka  and  near  the  Corn- 
stock  lode,  and  in  California.  The  so-called  quartz-porphyries  have 
been  also  met  in  many  Western  districts,  but  are  of  especial  im- 
portance at  Leadville,  where  they  are  intimately  associated  with 
the  ores.  The  ancient  rhyolites  (quartz-porphyries)  have  also  an 


26  A   HANDBOOK  OF  ROCKS. 

important  development  on  Lake  Superior.  The  greater  part  of 
the  boulders  in  the  Calumet  copper  conglomerate  consist  of  them, 
and  Lighthouse  Point,  near  Marquette,  furnishes  an  outcrop.  Along 
the  Atlantic  Coast  the  pre-Cambrian  rhyolites  (felsites)  are  present 
in  the  same  localities  as  those  cited  for  volcanic  glasses.  Recent 
rhyolites  are  in  vast  quantity  in  Iceland.  Many  are  known  in 
Europe,  but  the  enormous  development  in  Hungary  is  especially 
worthy  of  note.  The  sheets  of  rhyolite  on  the  Lipari  Islands,  be- 
tween Naples  and  Sicily,  suggested  the  name  Liparite.  In  almost 
all  volcanic  districts  they  are  liable  to  occur.  In  the  Tyrolese 
Alps  quartz-porphyries  are  ot  great  extent,  and  in  Scandinavia 
and  in  Cornwall,  they  form  important  dikes,  familiar  to  all  students 
of  the  subject. 

RHYOLITE  TUFFS. — These  are  the  fragmental  ejectamenta  from 
explosive  eruptions  that  often  afford  very  extensive  strata  of  rock. 
Although  loose  at  the  time  of  falling,  they  may  become  consolidated 
in  the  course  of  time,  or  before  this  occurs  they  may  be  sorted  and 
redeposited  in  water  so  as  to  share  the  nature  of  a  true  sediment. 
Fragments  of  volcanic  glass  and  of  all  the  component  crystals  of 
rhyolite  make  them  up,  while  larger  fragments  of  rock  and  vol- 
canic bombs  are  at  times  intermingled.  Tuffs  of  ancient  geologi- 
cal date  become  metamorphosed  and  recrystallized,  so  as  to  afford 
products  not  to  be  easily  distinguished  from  compact  felsites. 

Rhyolite  tuffs  are  abundant  along  the  eastern  foothills  of  the 
Front  Range  of  Colorado,  and  are  extensively  quarried  for  a  rather 
soft  building  stone. 

THE  TRACHYTES. 


Si02 

A12O3 

Fe2O3 

FeO 

CaO 

MgO 

K20 

Na20 

Loss. 

Sp.  Gr. 

I. 

66.03 

18.49 

2.18 

O.22 

0.96 

0-39 

5.86 

5.22 

0.85 

2-59 

2. 

65.07 

16.13 

5.17 

.     . 

2.74 

0.67 

4.44 

4-77 

0.70 

3- 

62.28 

19.17 

3-39 

1.44 

5-93 

5-37 

2-33 

2.65 

4- 

62.17 

18.58 

2-15 

1.05 

J-57 

°-73 

3.88 

7.56 

1.70 

5 

58.70 

19.26 

3-37 

0.58 

1.41 

0.76 

4-53 

8.55 

2.64 

6. 

57-7 

17.9 

4-4 

3-9 

3-7 

1.8 

7-7 

3-8 

O.I 

2.61 

I.  Trachyte,  Game  Ridge,  Custer  Co.,  Col.,  Cross,  Proc.  Col.  Sci.  Soc.,  1887,  237. 
2.  Oligoclase-trachyte  Drachenfels  on  Rhine,  Rammelsberg,  Z.  d.  d.  g.  G.  XL,  440, 
1859.  3.  So-called  Bostonite  dike,  Lake  Champlain,  J.  F.  Kemp,  Bull.  107,  U.  S.  G. 
S.,  20.  4-5.  Acmite-trachyte,  Crazy  Mountains,  Mont.,  Wolff  and  Tarr,  Bull.  Mus. 
Comp.  Zool.  XVI.,  232.  6.  Trachyte,  Arso  Flow,  Ischia  near  Naples.  Abich,  Isola 
dTschia,  38.  Silica  determinations  on  eleven  trachytes  from  the  Black  Hills  afforded 
J.  H.  Caswell  values  from  65.46  to  52.02. 


THE   TRACHYTES.  27 

Comments  on  the  Analyses. — The  decrease  in  silica  and  the  in- 
crease in  alumina  and  the  alkalies  as  against  the  rhyolites  is  note- 
worthy. The  alkalies  in  particular  are  high,  with  sometimes  pot- 
ash, sometimes  soda,  in  excess.  The  latter  marks  the  passage  to 
the  phonolites. 

Varieties.  Trachyte  is  a  name  derived  from  the  Greek  word  for 
rough,  and  refers  to  the  rough  surfaces  of  those  first  studied. 
The  name  was  of  much  wider  application  in  earlier  years  than  to- 
day and  was  used  for  both  rhyolites  and  trachytes.  If  soda  is 
high,  the  soda  pyroxene  acmite  may  form  and  give  name  to  the 
rock  as  in  analyses  4  and  5.  Pantellerite  is  another  name  for  those 
that  are  rich  in  soda  and  that  have  anorthoclase  feldspar.  The 
widely  and  loosely  used  name  porphyry  is  applied  to  pre-Tertiary 
trachytes,  and  to  intruded  sheets  and  dikes.  Bostonite  has  been 
employed  for  such  dikes  and  to  both  dikes  and  sheets  when  rich 
in  soda,  the  name  keratophyre  ha?  been  given.  Many  felsites  also 
belong  here,  but  for  all  these  the  remarks  made  under  rhyolites  ap- 
ply. The  .distinctions  are  only  of  importance  in  close  micro- 
scopical work  and  when  the  idea  of  the  geological  relations  is  in- 
volved in  the  definition,  but  both  porphyry  and  felsite  are  useful 
current  terms.  Porphyry,  it  should  be  remarked,  is  employed  in 
mining  circles  in  the  West,  for  almost  every  rock  that  occurs  in 
dikes  or  sheets. 

Mineralogical  composition.  The  principal  minerals  of  recognizable 
size  are  alkali-feldspar,  chiefly  orthoclase  of  the  variety  called  sani- 
dine,  a  little  acidic  plagioclase ;  more  or  less  biotite,  hornblende 
and  pyroxene  in  this  general  order.  The  light-colored  silicates  are 
in  notable  excess  over  the  dark  ones.  The  groundmass  is  glassy 
or  finely,  or  (rarely)  coarsely  crystalline. 

Alteration.  The  alteration  is  practically  the  same  as  that  de- 
scribed under  rhyolites. 

Relationships.  With  increase  of  soda,  trachytes  pass  into  phono- 
lites, to  which  indeed  they  are  closely  related.  With  the  develop- 
ment of  granitoid  texture  they  (including  porphyries)  pass  into 
syenites.  They  are  easily  confused  with  some  andesites  unless  the 
eye  can  detect  the  striated  feldspar,  but,  as  noted  in  the  next  para- 
graph, they  are  comparatively  rare  rocks. 

Distribution. — True  volcanic  trachytes  are  extremely  rare  in  this 
country,  for  many  of  the  earlier  cited  localities,  as;  for  instance, 
some  of  those  in  the  Reports  of  the  Fortieth  Parallel  Survey,  have 


28  A   HANDBOOK  OF  ROCKS. 

been  shown  to  be  andesites.  Beautiful  examples  do,  however,  oc- 
cur in  the  Black  Hills,  with  superbly  developed  orthoclases.  Others 
are  known  in  Custer  county,  Col.  (see  Analysis  i),  and  in  Montana 
(Analyses  4  and  5).  The  porphyries,  strictly  so-called,  are  not 
identified  with  certainty  in  very  wide  distribution,  although,  doubt- 
less, many  dikes  in  the  West  are  properly  described  as  such.  In 
southeast  Missouri,  at  Iron  Mountain  and  Pilot  Knob,  they  are 
very  abundant.  Many  curious  dikes  occur  around  Lake  Cham- 
plain,  and  among  the  pre-Cambrian  volcanics  of  the  Atlantic  Coast 
they  are  not  lacking.  Abroad  trachytes  are  more  common,  and 
along  the  Rhine, — where  the  peak  of  the  Drachenfels  is  situated, 
which  furnishes  the  commonest  specimens  for  collections, — in  the 
Auvergne,  in  Italy  and  in  the  Azores  they  are  well  known. 

Trachyte  Tuffs  are  not  common  in  America,  and  offer  only  micro- 
scopic points  of  difference  from  those  formed  of  rhyolitic  material. 

THE  PHONOLITES. 

SiO2       A12O3       Fe2O3         FeO      CaO      MgO     K2O      Na2O      Loss    Sp.  Gr. 


I. 

61.08 

18.71 

1,91 

0.63 

1.58 

0.08 

4.63 

8.68 

2.21 

2.582 

\ 

2. 

60.02 

20.98 

2.21 

0.51 

1.18 

tr. 

5-72 

8.83 

0.70 

2.576 

3- 

59-46 

23.00 

3-52 

I.OO 

0.50 

4.90 

7-13 

0.71 

4. 

59-17 

19.74 

3-39 

.   . 

0.92 

0.15 

6-45 

8.88 

1.18 

2.566 

5- 

56-43 

22.25 

2.66 

0.97 

1.41 

tr. 

2-7? 

II.  12 

2.05 

2-54 

6. 

49-18 

20.65 

.    . 

5-97 

2-43 

0.29 

6.88 

9.72 

i.  60 

2-553 

7- 

45.18 

23-31 

6.  1  1 

. 

4.62 

1.45 

5-94 

11.17 

1.14 

8. 

44-5° 

22.96 

6.84 

.   . 

8.65 

1.65 

4-83 

6.70 

2.06 

i.  Devil's  Tower,  near  Black  Hills,  Wyo.  Pirsson,  A.  J.  S.,  May,  1894,  344.  2.  El 
Paso  Co.  Colo.  Cross.  Proc.  Col.  Sci.  Soc.,  1887,  169.  3.  Island  of  Fernando  de  No. 
ronha,  Brazil,  Gumbel  Tscher,  Mitt.,  1880,  II.,  188.  4.  Near  Zittau,  Saxony,  v. 
Rath.  Z.  d.  d.  g.  G,  VIII.  297.  5.  Wolf  Rock,  Cornwall,  Eng.,  Phillips,  Geol.  Mag. 
VIII.  249.  6.  Leucite-phonolite,  near  Rieden,  Germany,  Zirkel,  Lehrbuch  II.,  465. 

7.  Eleolite-porphyry,   Beemerville,  N.  J.,  J.  F.   Kemp,    N.   Y.    Acad.   Sci.,  XL,  69. 

8.  Eleolite-porphyry,  Magnet  Cove,  Ark.,  J..F.  Williams,  Igneous  Rocks  of  Ark.,  261. 

Comments  on  the  Analyses. — It  is  at  once  apparent  from  the 
analyses  that  the  range  in  silica,  except  in  the  last  two,  is  much 
like  that  of  the  trachytes,  but  that  the  alumina  goes  higher,  and 
that  the  alkalies  are  in  extremely  large  amounts.  No  other  rocks, 
except  the  corresponding  granitoid  types,  reach  these  amounts  in 
alkalies.  The  soda  which  is  necessary  for  the  formation  of  the 
nepheline  is  naturally  in  excess.  The  rare  leucite-phonolites,  as  a 
general  thing,  are  more  basic  and  show  comparatively  high  potash. 


THE  PHONOLITES.  29 

The  last  two  analyses  of  intrusive  or  dike  members  are  abnormally 
basic  for  phonolitic  rocks. 

Varieties. — The  phonolites  were  named  because  in  the  thin  plates 
in  which  they  often  break  up  they  ring  under  the  hammer.  Neph- 
eline  or  eleolite-porphyries  have  been  described  by  a  few  obser- 
vers, and  certain  ones  in  dikes  have  been  named  tinguaite  from  a 
Brazilian  locality,  but  in  their  mineralogical  composition  they  are 
practically  phonolite,  although  at  times  exceptionally  basic.  Leu- 
citophyr  is  applied  to  phonolites  with  both  nepheline  and  leucite, 
while  leucite  phonolite  is  used  for  those  that  have  no  nepheline. 

Mineralogical  Composition. — The  principal  minerals  are  ortho- 
clase,  variety  sanidine,  and  nepheline  or  leucite,  or  both.  The 
nepheline  is  seldom  visible  to  the  eye,  and  indeed  it  is  practically 
necessary,  in  order  to  assure  oneself  of  the  rock  to  warm  up  a 
little  of  it  powdered,  in  dilute  acid,  and  evaporate  for  gelatinous 
silica.  Nepheline  gelatinizes  so  readily  that  it  is  easily  detected. 
Nosean  and  hauyne  are  frequent  in  phonolites.  The  commonest 
of  the  dark  silicates  are  the  soda-pyroxenes,  acmite  and  c-Egirine. 
Hornblende  is  known,  but  biotite  is  seldom  seen.  The  rocks  have 
usually  a  compact  gray  or  green  groundmass  in  which  are  visible, 
shining  sanidines,  seldom  nepheline  or  leucite,  and  rarely  dark  rods 
of  pyroxene,  because  the  pyroxene,  though  always  present,  is 
usually  microscopic.-  In  connection  with  pre-Tertiary  rocks  the 
name  eleolite  is  sometimes  used  for  nepheline. 

Relationships. — The  phonolites  are  closely  related  to  the  trachytes 
as  already  stated.  They  have  also  intimate  connections  with  cer- 
tain rare,  basaltic  rocks  to  be  referred  to  later'  that  contain  nephe- 
line and  leucite. 

Alterations. — The  nepheline  changes  quite  readily  to  natrolite 
and  perhaps  analcite,  while  leucite  yields  analcite.  Metamorphic 
processes  are  yet  to  be  studied. 

Distribution. — The  true  volcanic  phonolites  are  only  known  as 
yet  in  two  localities  in  this  country,  the  Black  Hills,  where  they 
form  dikes,  sheets  and  isolated  buttes  (Devil's  Tower),  and  the 
Cripple  Creek  mining  district  of  Colorado,  where  the  comparatively 
few  dikes  known  have  proved  of  great  importance  as  associates  of 
the  ores.  Nepheline-  or  eleolite-porphyries  (tinguaites)  are  recorded 
as  exceedingly  rare  rocks  near  Magnet  Cove,  Ark.,  and  Beemer- 
ville,  N.  J.,  associated  with  nepheline-syenite.  Phonolites  are 
much  more  abundant  abroad,  being  well  known  in  many  parts  of 


30  A   HANDBOOK  OF  ROCKS. 

Germany.  The  varieties  with  leucite  are  especially  familiar  from 
the  vicinity  of  Rieden,  in  the  extinct  volcanic  district  of  the  Eifel.  A 
peculiar  leucite  rock,  with  abundant  scales  of  biotite,  gives  the 
name  to  the  Leucite  Hills,  two  or  three  miles  north  of  Point  of 
Rocks,  Wyo.  Leucite  tinguaites  occur  near  Magnet  Cove,  Ark., 
in  the  High  wood  Mountains,  Mont.,  and  near  Rio  Janeiro,  Brazil. 
Tuffs  are  known  abroad  but  not  in  this  country,  and  exhibit  few 
features  calling  for  special  mention. 


THE  GRANITES. 


SiO2 

A12O3      Fe2O3      FeO 

CaO 

MgO 

K2O 

Na20 

Loss. 

Sp.  Gr. 

I. 

73.76 

13-43 

i, 

,16 

1.42 

0.75 

5.22 

4.01 

0.42 

2.63 

2. 

73-70 

14.44 

0.43 

1.49 

1.08 

tr. 

4-43 

4.20 

0.40 

2.69 

3- 

73-05 

14-53 

2.96 

.    . 

2.06 

tr. 

5-39 

i-73 

0.29 

4- 

72.73 

16.95                •    • 

1.05 

tr. 

8.15 

0.90 

O.22 

5- 

72.26 

15-59 

1.  16 

2.18 

1.13 

0.06 

5.58 

3-85 

0-47 

2.65 

6. 

71.78 

14-75 

.    . 

1.94 

2.36 

0.71 

4.89 

3.12 

0.52 

7- 

71.64 

15.66 

2.34 

.    . 

2.70 

tr. 

5.6 

1.58 

0.48 

8. 

69.46 

17-5° 

2.30 

.    . 

2-57 

0.30 

4.07 

2-93  vr 

0.82 

2.687 

9- 

69.28 

17.44 

2.30 

2.30 

0.27 

2.76 

3-64 

.     . 

10. 

68.68 

16.28 

0.66 

2-55 

2.24 

0.8  1 

4.07 

2.88 

0.85 

n. 

66.84 

18.32 

2.27 

0.20 

3.31 

0.81 

2.80 

5-14 

0.46 

12. 

66.68 

14-93 

1.58 

3-23 

4.89 

2.19 

2.05 

2.65 

1-25 

13- 

66.40 

'7.13 

.    . 

3-77 

4.05 

0-94 

2.08 

4-49 

1.03 

I.  Biotite  granite,  Green's  Landing,  Me.,  E.  F.  Hicks.  Privately  communicated. 
2.  Granitite,  Peterhead,  Scotland,  Phillips,  Q.  J.  G.  S.,  XXXVL,  1880,  13.  3.  Red 
Granitite,  Westerly,  R.  I.,  F.  W.  Love,  for  J.  F.  K.,  unpublished.  4.  Red  Granite, 
Stony  Point,  Conn.,  L.  P.  Kinnicut,  Anal.,  unpublished.  5.  Albany  granite,  N.  H 
Hornblende  granite,  G.  W.  Hawes,  A.  J.  S.,  Hi.,  XXL,  25.  6.  Hornblende  granite 
with  biotite,  Cottonwood  Canon,  Utah,  T.  M.  Drown,  4Oth  Parallel  Surv.,  L,  no.  7. 
Gray  granitite,  Westerly.  R.  L,  see  No.  3.  8.  Typical  granite,  Chester,  Mass.,  L.  M. 
Dennis,  for  J.  F.  K.,  N.  Y.  Acad.  Sci.,  XL,  129.  9.  Biotite  granite,  Raleigh,  N.  C, 
G.  P.  Merrill,  Stones  for  building  and  decoration,  418.  10.  Biotite  granite  with  horn- 
blende, Wood  Cone,  Eureka  Dist.,  Nev.,  A.  D.  Hague,  Mono.,  XX.,  U.  S.  G.  S.,  228. 
n.  Augite-soda  granite,  Kekequabic  Lake,  Minn.,  U.  S.  Grant,  Amer.  Geol.,  June, 
1893,  3^5-  I2-  Granitite,  Rowlandville,  Md.,  Jour.  Cin.  Soc.  Nat.  Hist,  1894,  p.  32. 
13.  Biotite  granite  with  hornblende,  El  Capitan,  Yosemite,  see  No.  6. 

Comments  on  the  Analyses. — These  analyses  illustrate  the  ge  al 
range  of  SiQ2,  but  granites  are  known  outside  of  both  limits.  is 
SiO2  decreases  the  bases  increase,  and  soda  tends  to  exceed  potash, 
marking  the  passage  to  the  diorites.  Those  high  in  Na2O,  like 
No.  II,  are  often  called  soda-granites.  They  are  analogous  to  the 
keratophyres,  soda-rhyolites  and  pantellerites,  earlier  referred  to. 


THE   GRANITES.  31 

The  whole  table  is  a  close  parallel  to  that  of  the  rhyolites.  The 
analyses  are  selected,  so  far  as  possible,  to  represent  prominent 
building  stones. 

Mineralogical  Composition  and  Varieties. — Granites  are,  par  excel- 
lence, granitoid  rocks  consisting  of  orthoclase,  sometimes  micro- 
cline,  some  acid  plagioclase,  quartz,  and  in  the  typical  variety  both 
biotite  and  muscovite.  Magnetite,  apatite  and  zircon  are  always 
present,  though  small,  and  garnet  is  not  at  all  unusual.  Biotite  is 
much  the  commoner  of  the  micas,  and  when  it  is  present  alone  the 
rock  is  sometimes  called  granitite.  Granites  with  muscovite  alone 
are  especially  found  in  the  form  of  dikes.  They  are  called  .aplite. 
Hornblende  is  also  frequently  met,  either  with  biotite  or  by  itself, 
giving  then  hornblende-granite.  In  former  years  this  aggregate 
was  called  syenite,  but  the  modern  usage  is  different.  Augite  in 
granites  is  uncommon,  and  marks  a  passage  to  the  gabbros.  All 
forms  of  dark  silicates  and  mica  may  fail,  and  then  we  have  the 
so-called  binary  granites.  Some  Missouri  granites  are  of  this 
character. 

Especially  in  regions  of  granite  intrusions  and  of  extensive 
metamorphism,  veins  or  dikes— (it  is  an  open  question  which  is 
the  more  correct  term — are  met  formed  of  very  coarsely  crystalline 
aggregates  of  the  same  minerals  that  constitute  granite.  These 
are  called  pegmatite  and  in  them  is  the  home  of  graphic  granite, 
the  curious  intergrowth  of  quartz  and  feldspar,  such  that  a  cross 
fracture  of  the  blades  of  quartz  suggests  cuneiform  characters. 
Garnet,  tourmaline,  beryl  and  minerals  involving  the  rare  earths, 
are  often  found  in  pegmatites,  and  they  supply  the  feldspar  and 
mica  of  commerce.  The  outcrops  may  be  two  hundred  feet 
broad  or  more,  and  again  the  same  aggregates  a're  found  as  small 
lenses  or  "  Augen  "  in  metamorphic  rocks.  In  regard  to  the  larger 
veins  or  dikes  it  seems  improbable  that  true  igneous  fusion  could 
have  afforded  such  coarsely  crystalline  aggregates,  and  so  we  are 
rorced  to  assume  such  abundance  of  steam  and  other  vapors,  i.  e., 
Mneralizers,  as  to  almost,  if  not  quite,  imply  solution. 

Jndoubted  dikes  of  the  composition  of  granite  are  also  known, 
.  .at  have  no  such  unusual  size  of  minerals,  but  that  tend  to  de- 
velop a  porphyritic  texture  from  the  presence  of  feldspars  larger 
than  the  general  run  of  the  component  minerals.  These  are  called 
granite-porphyries,  and  they  pass  by  insensible  gradations,  through 
finer  and  finer  groundrnasses,  into  typical  quartz-porphyries.  The 


32  A   HANDBOOK  OF  ROCKS. 

phenocrysts  of  granite-porphyries  may  also  fail  and  the  ground- 
mass  may  become  finer  and  finer,  passing  through  a  stage  called 
micro-granite  into  the  felsites. 

The  outer  portions  of  granite  masses  are  often  subjected  to  the 
action  of  escaping  vapors,  containing  boracic  and  hydrofluoric  acids 
(fumarole  action).  These  develop  tourmaline  in  quantity  and  often 
fluorite,  and  in  rare  instances  cassiterite.  In  a  famous  case  near 
Luxullian,  in  Cornwall,  the  feldspar  has  become  changed  to  an  ag- 
gregate of  tourmaline  needles  and  quartz,  and  the  rock  is  called 
luxullianite.  Tourmaline  granite  is,  however,  also  known  in  which 
tourmaline  plays  the  role  of  mica  or  hornblende,  as  at  Predazzo,  in 
the  Tyrol.  Fumarole  action  may  change  the  borders  of  granites 
to  a  mass  of  quartz  and  a  lithia  mica,  affording  the  rock  that  is 
called  greisen  and  that  is  a  familiar  gangue  for  tin  ores. 

Granites  are  commonly  gray,  bluish  or  reddish  in  color.  The 
feldspar  is  mainly  responsible  for  this,  as  quartz  is  colorless  and 
transparent  and  biotite  and  hornblende  are  not  specially  abundant ; 
but  unusual  richness  in  the  last  named  silicates  tends  to  darken  the 
shade.  These  latter  are  very  frequently  segregated  into  the  black 
bunches  that  are  noticeable  in  many  building  stones.  They  may  be 
spheroidal  in  their  alignment,  affording  so-called  orbicular  granite. 

Relationships. — The  passage  of  granites,  through  granite-por- 
phyries and  micro-granites,  into  quartz-porphyries  and  felsites,  has 
been  remarked.  Sometimes  along  the  border  of  an  intrusion,  this 
can  be  traced  inch  by  inch  to  a  place  where  the  porphyritic  tex- 
ture is  due  to  a  quick  chill.  Mt.  Willard,  in  the  Crawford  Notch 
of  the  White  Mountains  is  a  classic  locality  of  this  phenomenon. 
It  was  described  in  1881  by  Geo.  W.  Hawes  (see  analysis  6\  and 
will  be  referred  to  again  under  the  rocks  of  contact  metamorphism. 
The  close  relationship  of  the  granitoid  rhyolites  or  nevadites  with 
granite  need  only  be  referred  to.  As  quartz  decreases,  syenites 
result  by  insensible  gradations,  and  as  hornblende  or  biotite  and 
plagioclase  increase,  the  same  passage  is  made  to  diorites.  Inter- 
mediate varieties,  which  are  very  common,  are  often  called  granite- 
diorites  or  grano-diorites.  Transitional  passages  to  gabbro,  from 
increase  of  augite  and  plagioclase,  are  also  well  recognized. 

Geological  Occurrence. — Granites  in  their  most  typical  develop- 
ment c  nstitute  great  irregular  masses  that  have  solidified  at 
depths;  such  are  called  bathylites,  and  it  is  generally  believed  that 
before  consolidating  they  have  often  fused  their  way  upward  by 


THE  GRANITES.  33 

melting  into  themselves  overlying  rock.  Granites  also  appear  as 
irregular  or  rounded  outcrops  in  the  midst  of  other  rocks  (bosses 
or  knobs)  and  as  dikes.  There  is  no  reason  why  granites  should 
not  form  at  all  geological  ages,  but  those  open  to  our  observation 
are  mostly  Archean  and  Paleozoic  because^  being  deep-seated 
rocks,  only  the  older  ones  have  been  exposed  by  erosion.  The 
relations  of  pegmatites  to  veins  have  been  earlier  set  forth.  Gran- 
ites tend  to  break  apart  by  jointing  planes  in  rectangular  blocks, 
a  property  that  much  facilitates  their  quarrying.  They  also  have 
lines  of  weakness  admitting  of  their  further  division  into  smaller 
masses.  The  development  of  these  is  more  or  less  characteristic  of 
each  particular  locality. 

Uses. — Granites  are  much  more  extensively  employed  for  struc- 
tural purposes  than  any  other  igneous  rock,  and  indeed  in  the 
trade  any  crystalline  rock  consisting  of  silicates  is  called  granite. 
They  are  in  general  the  strongest  of  the  common  building  stones. 
Crushing  resistances  range  from  10,000  to  25,000  pounds  per 
square  inch  in  a  2-inch  cube.  The  important  points  are  homo- 
geneity of  texture,  good,  rectangular  cleavages  in  the  quarry, 
adaptability  to  tool  treatment,  and  pleasing  color. 

Alteration,  Metamorphism . — In  ordinary  decay  granites  suffer 
first  by  the  oxidation  of  the  protoxide  of  iron  in  the  ferromagnesian 
silicates  (biotite,  hornblende),  and  the  formation  of  chlorite  and 
other  secondary  minerals.  The  feldspars  also  kaolinize,  and  the 
rock  thus  becomes  hydrated.  Pyrite,  if  present,  is  an  active  agent 
in  decay.  Yet  the  chemical  changes  involved,  except  hydration, 
seem  to  be  comparatively  slight  even  in  the  change  from  granite 
to  soil.  G.  P.  Merrill  gives  the  following  analyses  of  unaltered 
and  altered  biotite  granite  from  the  vicinity  of  Washington,  D.  C. 
(Bull.  Geol.  Soc.  Amer.  VL,  323). 

SiO2        A12O3        Fe2O3        FeO        CaO        MgO         K2O        Na2O  Ignition. 

1.  69.33        H-33  •    .  3-6°         3-2i         244         2.67          2.70          1.22 

2.  66.82       15.62          1.88          1.69        3.13        2.76         2.04         2.58          3.27 

3.  65.69          15.23  4.39  2.63  2.64  2.00  2.12  4.70 

No.  I  is  fresh  and  undecomposed  rock ;  No.  2,  decomposed  but 
still  moderately  firm  rock  ;  No.  3,  soil.  It  is  evident  at  once  that 
there  has  been  considerable  hydration,  and  that  a  notable  decrease 
in  the  alkalies  has  occurred,  each  being  affected  about  equally  in 
the  end,  although  K2O  yields  first;  MgO  has  relatively  increased  ; 
CaO  has  suffered  loss;  the  FeO  is  all  oxidized,  the  A12O3  has  rela- 

3 


34  A'  HANDBOOK  OF  ROCKS. 

lively  increased  and  the  SiO2  decreased.  While  appreciating  these 
chemical  changes,  Dr.  Merrill  still  emphasizes  the  much  greater 
importance  of  the  physical  alteration  and  attributes  this  to  swelling 
from  hydration.  Other  interesting  data  are  given  in  the  citation. 
Similar  sets  of  parallel  analyses  have  been  made  abroad  with  analo- 
gous results  in  the  case  of  the  chemical,  rearrangements. 

Under  dynamic  stress  granites  are  more  or  less  crushed  and 
have  their  minerals  drawn  out  into  laminations  from  shearing 
strains  so  that  they  readily  assume  gneissoid  structures.  Beyond 
question  many  gneisses  have  resulted  in  this  way,  and  in  the 
geology  of  some  districts,  as,  for  instance,  the  Front  Range  of 
Colorado,  we  employ  the  term  granite- gneiss.  The  structures 
were,  doubtless,, induced  while  the  granite  was  deeply  buried  and 
subjected  to  pressure  while  closely  confined,  so  that  the  yielding 
came  in  a  gradual  flow. 

Distribution. — Granites  are  abundant  along  the  Atlantic  coast, 
and  are  near  tidewater  from  Canada  to  Virginia.  Further  south 
they  lie  back  of  the  Coastal  Plain.  They  are  chiefly  biotite  granite 
and  are  extensively  quarried.  A  famous  hornblende  granite  is 
obtained  at  Quincy,  Mass.,  that  was  formerly  called  syenite.  In 
the  old  crystalline  areas  of  Michigan,  Wisconsin  and  Minnesota 
they  are  common.  Missouri  has  many  in  the  region  of  the  por- 
phyries, already  cited.  In  the  West,  the  Black  Hills,  the  Rocky 
Mountains,  the  Wasatch  and  the  Sierras  are  abundantly  supplied. 
They  are  equally  common  in  Europe  and  elsewhere  the  world 
over. 


THE  SYENITES. 

Si02       A1203      Fe203      FeO        CaO      MgO       K2O      Na2O      Loss     Sp.  Gr. 
60.03       20.76       4.01        0.75        2.62       0.80       5.48       5.96       0.59 


2. 

59.83 

16.85 

7.01 

4-43 

2.61 

6.57 

2-44 

1.29 

2-73 

3- 

59.78 

16.86 

3.08 

3.72 

.   2.96 

0.69 

5.01 

5-39 

1.58 

2.689 

4- 

59-37 

17.92 

6.77 

2.02 

4.16 

1.83 

6.68 

1.24 

0.38 

2.71 

5- 

56-45 

20.08 

1.31 

4-39 

2.14 

0.63 

7.13 

5.61 

1.77 

6. 

46.11 

H-75 

2.20 

4-51 

7-82 

5-73 

3-84 

1.29 

i-59 

2.904 

7.      46.73       10.05       3.53      8.20       13.22       9.68      3.76       1.81        1.24 

I.  Fourche  Mtn.,  near  Little  Rock,  Ark.,  J.  F.  Williams;  Igneous  Rocks  of  Ark., 
88.  2.  Plauen,  near  Dresden,  F.  Zirkel,  Pogg.  Ann.,  CXXIL,  622.  3.  Custer  Co., 
Colo.  Cross,  Proc.  Colo.  Sci1.  Soc.,  1887,  240.  4.  Biella,  Piedmont,Cossa.,  Turin  Acad. 
ii.,  XVIII.,  28.  5.  Sodalite-Syenite  Highwood  Mtns.,  Mont.  W.  Lindgren,  A.  J.  S., 
Apr.,  1893,  296.  6.  Minette,  Rhode  Island,  badly  decomposed,  contained  CO2  7.32, 


THE  SYENITES.  35 

Pirsson,  A.  J.  S..  Nov.,  93,   375.     7.  Shonkinite,  Highwood  Mtns.,    Mont,  Pirsson, 
Bull.  Geol.  Soc.  Amer.  VI.  414. 

Comments  on  the  Analyses.  The  syenites  mark  a  decrease  in 
SiO2  from  the  granites  and  a  general  increase  in  all  the  bases  The 
high  percentage  of  alkalies  is  especially  worthy  of  remark,  and  the 
notably  large  amounts  of  soda,  showing  the  passage  to  the  nephe- 
line  syenites.  The  parallelism  with  the  trachytes  is  close.  The 
last  two  analyses  exhibit  excessively  basic  extremes,  whose  theo- 
retical significance  is  commented  on  in  the  next  paragraph. 

Mineralogical  Composition,  Varieties. — The  name  syenite  was 
suggested  by  Syene,  now  Assuan,  an  Egyptian  locality  where  a 
hornblende  granite  was  formerly  obtained  for  obelisks,  and  if  its 
local  significance  were  perpetuated,  syenite  as  formerly  should  be 
applied  to  this  rock.  But  Werner  used  it  in  the  last  century  for 
the  well-known  typical  rock  from  the  Plauenschen  Grund  (see 
Analysis  2 j,  near  Dresden,  that  contains  almost  no  quartz,  and  .of 
recent  years  this  has  been  its  correct  use.  Typical  syenites  have 
orthoclase  and  hornblende ;  those  with  biotite  are  called  mica- 
syenites.  Some  plagioclase  is  always  present  and  magnetite, 
apatite  and  zircon  are  invariable.  Mica  syenites  in  dikes,  basic 
and  of  dark  color  have  been  called  minette.  Orthoclase  and  augite 
afford  augite-syenite.  An  excessively  basic  one  (Analysis  7),  from 
the  Highwood  mountains,  Mont.,  has  recently  been  described  by 
Pirsson  under  the  name  Shonkinite.  It  is  of  great  theoretical  im- 
portance, as  it  shows  that  orthoclase  is  not  limited  to  acidic  rocks, 
but  may  be  the  prevailing  feldspar  in  very  basic  ones.  Still  more 
recently  J.  P.  Iddings  has  noted  others  of  similar  character  from 
the  region  of  the  Yellowstone  Park.  (Jour,  of  Geology,  Decem- 
ber, 1895,935.)  Basic  nepheline-syenites  have  been  earlier  known. 
Still  th.e  table  on  page  55  expresses  the  general  truth,  the  excep- 
tions being  excessively  rare  rocks  so  far  as  yet  known.  Syen- 
ites are  themselves  rare  rocks.  With  high  soda,  the  mineral  sodal- 
lite  develops  and  yields  sodalite  syenites  which  are  passage  forms 
to  nepheline  syenites. 

Relationships. — Syenites  are  most  closely  allied  with  nepheline- 
syenites,  into  which  with  increase  of  soda  they  readily  pass.  They 
also  with  increasing  plagioclase  shade  into  diorites  and  the  augite- 
syenites  are  closely  akin  to  gabbros. 

Geological  Occurrence. — Syenites  form  irregular  masses  and  dikes, 
precisely  as  do  granites. 


36  A   HANDBOOK  OF  ROCKS. 

Alteration. — There  is  little  to  be  said  that  was  not  covered  under 
granite.  The  rarity  of  syenite  makes  it  a  much  less  serious  factor. 
In  metamorphism  they  pass  into  gneisses. 

Distribution — Syenites  occur  in  the  great  igneous  complex  of  the 
White  Mountains.  They  form  large  knobs  and  dikes  near  Little 
Rock,  Ark.,  and  a  dike  is  known  in  Custer  county,  Colo.  The 
only  American  minette  yet  discovered,  is  a  dike  on  Conanicut 
Island,  R.  I.,  described  by  Pirsson  (see  Analysis  7).  Abroad,  sye- 
nites are  better  known.  The  Plauenscher  Grund,  near  Dresden, 
Biella  in  the  Piedmont,  and  the  vicinity  of  Christiania,  Norway, 
are  the  best  known.  Minettes  are  especially  famous  in  connection 
with  the  mining  district  about  Freiberg,  Saxony,  and  in  the  Vos- 
ges  mountains. 

THE  NEPHELINE  SYENITES. 

Sp.  Gr. 


SiO2 

MA 

Fe2O3 

FeO 

CaO 

MgO 

K20 

Na20 

Loss. 

/I. 

60.39 

22.51 

0.42 

2.26 

0.32 

0-13 

4-77 

8.44 

o-57 

2. 

59.70 

18.85 

4.85 

.    . 

1-34 

0.68 

5-97 

6.29 

1.88 

3- 

59-01 

18.18 

1-63 

3-65 

2.40 

1.05 

5-34 

7-03 

0.50 

4- 

56.30 

24.14 

1.99 

.    . 

0.69 

0.13 

6.79 

9.28 

1.58 

5- 

54.20 

21.74 

0.46 

2.36 

1.95 

0.52 

6.97 

8.69 

6. 

52-75 

22-55 

.    3-65 

.    . 

1.85 

0.15 

7-05 

8.10 

3.60 

7- 

•  S1^0 

22.54 

4-03 

3-15 

3.11 

1.97 

4.72 

8.18 

O.22 

8. 

50.96 

19.67 

7.76 

.    . 

4.38 

0.36 

6.77 

7.67 

1.38 

9- 

50.36 

19.34 

6.94 

.    . 

3-43 

.   . 

7.17 

7.64 

3-51 

10. 

41.37 

16.25 

16.93 

.    . 

i2-35 

4-57 

3-98 

4.18 

0.45 

i.  So-called  Nepheline-syenite,  or  Litchfieldite,  Litchfield,  Me.,W.  S.  Bayley,G.  S.  A., 
III.,  241.  2.  Nepheline-syenite,  Fourche  Mountains,  Ark.,  J.  F.  Williams,  Igneous 
Rocks  of  Ark.,  88.  3.  Nepheline-syenite,  Red  Mountains,  N.  H.,  W.  S.  Bayley,  G.  S. 
A.,  III.,  250.  4.  Ditroite,  Hungary,  Fellner,  Neues,  Jahrb.,  1868,  83.  5.  Foyaite,  Portu- 
gal, Jannasch.  Neues  Jahrb.,  II.,  II.  6.  Nepheline  syenite,  Sao  Paulo,  Brazil,  Ma- 
chado.  Tsch.  Mitt.,  IX.,  1888,334.  7.  Laurdalite,  variety  of  Nepheline-syenite.  Lund, 
Norway  jJBrogger,  Syenit-pegmatit-gange,  33.  8.  Leucite-syenite,  Arkansas,  J.  F.  Wil- 
liams. Igneous  Rocks  of  Ark.,  276.  9.  Nepheline-syenite,  Beemerville,  N.  J.,  F.  W. 
Love  for  J.  F.  K.,  N.  Y.  Acad.  Sci.,  XL,  66.  10.  Basic  Nepheline-syenite,  Beemerville, 
N.  J.,  J.  F.  Knmp,  N.  Y.  Acad.  Sci.,  XL,  86. 

Comments  on  the  Analyses. — A  considerable  range  is  shown  in  the 
SiO2,  some  analyses  going  below  the  usual  percentages  for  syenites 
and  the  last  analysis  being  abnormal.  In  general  the  amounts  of 
alkalies  are  extremely  high,  with  Na2O  in  excess,  in  which  re- 
spect the  phonolites  are  paralleled. 

Mineralogical  Composition  and  Varieties. — The  minerals  of  nephe- 


THE  NEPHELINE  SYENITES.  37 

line  syenite  are  in  general  the  same  as  those  of  syenite  proper, 
with  the  addition  of  nepheline,  often  sodalite,  and  several  charac- 
teristic ones  into  which  the  rare  earths  often  enter  as  bases.  Zir- 
con is  widespread  and  often  large  enough  to  afford  fine  crystals. 
For  this  reason  the  rocks  were  named  zircon-syenite  many  years 
ago.  The  nepheline  is  often  called  eleolite  (or  elaeolite),  from  the 
former  custom  of  speaking  of  this  mineral  in  pre-Tertiary  rocks  as 
elaeolite  and  in  later  ones  as  nepheline,  just  as  we  have  had  ortho- 
clase  and  sanidine,  but  the  custom  is  gradually  falling  into  disuse. 
Attempts  have  been  made  to  give  different  names  according  to  the 
dark  silicate ;  for  instance,  those  with  hornblende  were  called  foyaite, 
from  Foya,  a  Portuguese  locality;  those  with  biotite,  miascite  from 
Miask,  in  the  Urals.  But  both  these  minerals  so  often  appear  to- 
gether or  with  pyroxene  that  the  practice  is  not  generally  observed. 
Ditroite  is  a  variety  rich  in  blue  sodalite.  The  Litchfield,  Me., 
rock  has  been  shown  by  Bayley  to  have  as  its  feldspar  almost  ex  • 
clusively  albite,  and  he,  therefore,  has  called  it  litchfieldite.  The 
texture  of  nepheline-syenites  varies  very  much.  At  times  it  is 
very  coarsely  granitoid,  and  again  it  is  what  is  called  trachytic, 
i.  e.,  with  little  rods  of  feldspar,  more  or  less  in  flow  lines,  like  a 
trachyte  and  marking  a  passage  to  the  phonolites.  Types  have 
been  based  on  these  characters.  Where  at  all  finely  crystalline, 
the  determination  of  nepheline-syenites,  as  against  true  syenites, 
is  a  matter  for  the  microscope.  Nepheline-syenites  are  compara- 
tively rare  rocks.  Corresponding  rocks  with  leucite  are  as  yet 
only  known  from  Arkansas  and  Montana. 

Relationships. — As  already  remarked,  the  nepheline-syenites  are 
closely  related  to  the  true  syenites,  and  to  the  phonolites.  With 
certain  basic  plagioclase  rocks  with  nepheline,  called  theralites, 
they  are  also  of  near  kinship. 

Geological  Occurrence,  Alteration. — The  nepheline-syenites  are 
specially  prone  to  appear  as  dikes,  often  on  a  very  large  scale. 
Their  alteration  affords  no  special  features,  as  distinguished  from 
the  syenites  or  granites,  except  as  regards  the  secondary  minerals 
from  the  nepheline.  Natrolite,muscoviteand  kaolin  are  all  known 
in  this  relation  and  the  last  two  have  been  called  liebenerite  and 
gieseckite.  Cancrinite  also  results  from  the  alteration  of  nepheline. 
The  rarity  of  the  nepheline-syenites  has  prevented  their  playing 
an  important  role  among  metamorphosed  rocks. 

Distribution. — Nepheline-syenites  are  known  in  this  country  at 


38  A   HANDBOOK  OF  ROCKS. 

Montreal  and  Dungannon,  Ont;  Litchfield,  Me. ;  Red  Hill,  N.  H.; 
Salem,  Mass. ;  Beemerville,  N.  J.,  where  a  superb  dike  is  exposed  ; 
and  near  Little  Rock,  Ark.,  where  the  area  is  extensive.  Very  in- 
teresting ones  occur  near  Rio  Janeiro,  and  in  the  State  of  Sao 
Paulo,  Brazil.  Abroad  the  Portuguese  locality,  in  the  Monchique 
Mountains  ;  the  one  at  Ditro,  in  Hungary,  and  the  wonderful  dikes 
near  Christiania,  in  Norway,  so  prolific  in  rare  minerals,  are  of 
especial  interest. 


XIN-  SIT 


CHAPTER    IV. 

THE  IGNEOUS  ROCKS,  CONTINUED.     THE  DACITES,  THE  ANDESITES 
AND  THE  ROCKS  OF  THE  BASALT  GROUP. 

THE  DACITES  AND  ANDESITES. 


8. 


THE  DACITES. 

SiO2      A12O3      Fe2O3      FeO      CaO      MgO  Na2O  K2O  Loss.     Sp.  Gr. 

69.96      15.79      2.50         .    .        1.73       0.64  3.80  4.12  1.53 

69.36     16.23      0.88       1.53       3.17       1.34  4.06  3.02  0.45 

67.49      1 6. 18      1.30       1.22       2.68       1.34  4.37  2.40  2.69 


67.2  17.0        3.5 

67.03  16.27       .    . 


4-5 


3-7 


t.6 


0.9 


3-97      3-42      1.19      2.71       3.50       1.56 


66.03     14.57      2.57       1.19      3.38       1.89      3.71       2.70       2.07 
63-36     l6-35      2.12      3-°5      4-79      3-28       3.58       2.92       0.99 


67.83     15.02 


THE  ANDESITES. 

5.16     3.07     0.29     2.40      3.20 


9- 

65-5° 

14.94 

1.72 

2.27 

2-33 

2.97 

5-46 

2.76 

i-37 

10. 

63-49 

18.40 

2.44 

1.09 

2.30 

.66 

5-70 

4.62 

1.04 

n. 

62.94 

18.14 

.   . 

3.82 

6.28 

3.06 

3-83 

1.22 

0.60 

12. 

61.62 

16.86 

.   . 

6.61 

6.57 

2.07 

3-93 

1.66 

13- 

61.58 

16.34 

6.42 

S-13 

2.85 

2.69 

3-65 

0.64 

14. 

59-48 

16.37 

3.21 

3-17 

4.88 

3-29 

3-30 

2.81 

2.02 

IS" 

56.19 

16.12 

4.92 

4-43 

6.99 

4.60 

2.96 

2-37 

1.03 

16.        56.91      18.18       4.65       3.61       7.11       3.49      4.02       1.61       0.36 

i.  McClelland  Peak,  near  Comstock  Lode,  Nev.,  F.  A.  Gooch.,  Bull.  17,  U.  S.  G. 
S.,  33.     2.  Lassen's  Peak,  California,  Hague  and  Iddings,  A.  J.  S.,  Sept.,  1883,  232. 

3.  Sepulchre  Mountain,  Yellowstone  Park,  J.  P.  Iddings,  Phil.  Soc.  Wash.,  XL,  210. 

4.  Nagy-Sebes,    Hungary,  Doelter   Tscher.  Min.  Petr.  Mitt,  1873,  93.     5.    Eureka 
Dist,  Nev.     A.  Hague  Mono.,  XX.,  U.  S.   G.   S.,  264.     6-7.  Colombia,  S.  America. 
From  Kuch's  Petrographie  of  Colombian  Volcanoes,  quoted  in  Jour.  Geol.,  L,  171.     8. 
Hb.-mica-andesite,  Eureka  Dist.,  Nev.,  Mono.,  XX.,  U.  S.  G.  S.,  264.     9.  Hb.-mica- 
andesite,  Sepulchre  Mountain,  Yellowstone  Park,  J.  P.  Iddings,  Phil.  Soc.  Wash.,  XL, 
210.  Compare  No.  3.     10.  Mica-andesite,Rosita  Hills,  Colo.,  W.  Cross,  Colo.  Sci.  Soc., 
1887,  25°-   1 1-  Lassen's  Peak,  Calif.,  Hague  and  Iddings,  A.  J.  S.,  Sept.,  1883,  225.    12. 
Mt.  Rainier.  See  last  reference.   13.  Pyroxene-andesite,  Eureka  Dist.,  Nev.,  Mono.,  XX. 

39 


40  A   HANDBOOK  OF  ROCKS. 

U.  S.  G.  S.,  264:  Compare  No.  5.  14.  Hypersthene-andesite,  near  Red  Bluff, 
Mont.,  G.  P.  Merrill,  Proc.  U.  S.  Nat'l  Museum,  XVII.,  651.  15.  Hypersthene-ande- 
site, Buffalo  Peaks,  Colo.,  W.  Cross,  Bull  I.,  U.  S.  G.  S.,  26.  16.  Colombia,  S.  America. 
See  Nos.  6  and  7. 

Comments  on  the  Analyses.  It  appears  at  once  from  the  analyses, 
that  the  dacites  are  high  in  silica,  in  which  they  equal  the  lower 
ranges  of  rhyolites.  As  compared  with  the  latter,  soda  is  prevail- 
ingly in  excess  of  potash,  and  as  a  rule  the  other  bases  run  higher. 
The  andesites  lap  over* the  lower  limits  of  the  dacites  and  have 
much  the  same  range  in  silica  as  the  trachytes.  All  the  bases  reach 
notable  percentages,  but  the  alkalies  recede  as  the  others  increase. 

Mirier alogical  Composition,  Varieties.  The  name  dacite  is  derived 
from  the  old  Roman  province  of  Dacia,  now  a  part  of  modern 
Hungary  The  name  andesite  was  suggested  by  the  abundance  of 
these  lavas  in  the  Andes  Mtns.  Quartz  occurs  usually  in  abundant 
crystals  in  the  dacites,  causing  them  to  closely  resemble  the 
rhyolites.  The  distinction,  when  it  can  be  made,  and  this  is  not 
always  without  the  microscope,  depends  on  the  prevalence  of 
striated  feldspars.  The  prevailing  dark  silicate  is  biotite,  as  is 
usually  the  case  with  an  acidic  rock.  Hornblende  and  augite  are 
rarer.  Magnetite  and  the  small  accessory  minerals,  apatite,  zircon, 
etc.,  are  generally  present.  In  the  acidic  andesites,  biotite  is  also 
commonest ;  hornblende  and  then  pyroxene  favor  those  of  decreas- 
ing silica.  Andesites  with  biotite  are  usually  called  mica-andesites. 
Andesite  when  used  alone  implies  hornblende-ande^e.  Prevail- 
ing augite  is  indicated  by  the  name  augite-andesite.  If  magnesia 
is  in  considerable  amount  hypersthene  may  result  and  af- 
ford hypersthene-andesite,  a  frequent  rock  in  the  West,  but 
for  all  one  can  usually  say  from  ordinary  examination,  the 
rocks  may  be  augite-andesite,  or  even  hornblende-andesite. 
Andesites  strongly  resemble  trachytes,  but  it  is  to  be  appreciated 
that  trachytes  are  comparatively  rare  rocks,  while  andesites 
are  among  our  commonest  lavas,  and  along  the  west  coast  of 
North  and  South  America  are  the  prevailing  volcanic  rock.  The 
dark  silicates  are  also  increasingly  abundant  in  the  andesites.  They 
pass  insensibly  into  diorites,  by  the  development  of  granitoid  tex- 
ture. The  older  andesitic  lavas  have  been  called  porphyrites,  on 
the  analogy  of  porphyry,  and  then  mica,  or  hornblende  or  augite 
is  prefixed.  Others  use  porphyrite  for  intruded  sheets  and  dikes; 
and  others  still  for  varieties,  with  a  groundmass  of  medium  coarse- 


THE  DACITES  AND   ANDESITES.  41 

ness,  restricting  thus  andesite  to  the  finely  crystalline  or  glassy 
varieties  of  groundmiss  and  diorite  to  the  coarse,  granitoid  rocks. 
These  distinctions  belong,  however,  to  the  refinements  of  the  sub- 
ject. 

On  analogy  with  the  name  trachyte,  which  was  formerly  ap- 
plied in  the  field  to  all  these  more  or  less  rough  and  cellular  lavas, 
but  which  is  now,  by  universal  consent,  restricted  to  the  orthoclase 
rocks,  G.  F.  Becker  has  suggested  that  "  asperite"  be  used  for 
those  with  plagioclase,  basing  it  on  the  Latin  word  for  rough. 
With  general  acceptance  it  ought  to  prove  a  very  useful  term,  be- 
cause the  observer  is  often  in  doubt  whether  a  rock  is  dacite  o*r 
andesite,  and  if  the  latter,  to  which  group  it  belongs.  Propylite 
is  a  name  still  more  or  less  current  in  the  West.  It  was  created 
for  a  series  of  rather  coarsely  crystalline  or  granitoid  andesites,  that 
are  of^fe--Tertiary  age,  and  that  often  have  the  dark  silicates  altered 
to  secondary  minerals.  The  name  means  "  before  the  gates,"  and 
the  significance  was  that  coming  just  before  the  geological  time  of  the 
true  volcanics,  yet  resembling  them,  they  deserved  this  distinction. 
It  is  now  obsolete  and  reasons  for  its  special  existence  were  long 
ago  exploded,  but  having  been  employed  on  the  Comstock  lode,  it 
has  passed  into  western  usage.  There  is  no  special  and  neces- 
sary geological  age  for  any  igneous  rock. 

Alteration,  Met  amor phism. — The  andesites  in  decay  afford  kaoli- 
nized  material  and  mixtures  of  this  with  chloritic  products  that  are 
very  difficult  j  identify.  Thus  the  now  famous  andesitic  breccia 
at  Cripple  Creek,  Colo.,  can  rarely  be  shown  to  the  eye  to  be  other 
than  a  white,  kaolinized  mass,  and  decomposed  outcrops  of  mas- 
sive flows  are  no  less  unsatisfactory.  Where  metamorphic  pro- 
cesses affect  older  flows,  felsitic  and  silicified  forms,  result  similar 
to  those  -mentioned  under  rhyolites.  The  tracing  of  the  history 
of  the  rock  is  then  a  matter  for  the  microscope  and  chemical 
analysis,  when  indeed  it  can  be  done. 

Tuffs. — Andesitic  tuffs  and  breccias  (i.  e.,  aggregates  of  angular, 
volcanic  ejectments  coarser  than  tuffs)  are  rather  common  in  the 
western  volcanic  districts.  With  ordinary  observation  they  can 
only  be  identified  by  finding  fragments  large  and  fresh  enough  to 
indicate  the  original.  Such  have  proved  of  great  economic  im- 
portance at  Cripple  Creek,  Colorado. 

Distribution. — Andesites  are  very  wide-spread  in  the  West.  The 
vast  laccolites  that  form  many  of  the  peaks  in  Colorado  are  in- 


42  A    HANDBOOK  OF  ROCKS. 

truded  andesites  (porphyrites)  of  a  rather  acidic  type,  frequently 
with  some  orthoclase.  In  the  Yellowstone  Park  they  are  impor- 
tant. In  Nevada,  as  at  Eureka  and  the  Comstock  lode,  they  have 
proved  of  great  geological  interest,  and  especially  near  the  latter,  with 
its  many  miles  of  drifts,  shafts  and  tunnels,  very  important  data  for 
the  study  of  rock  masses  have  been  afforded.  The  old  cones  along 
the  Pacific,  Mt.  Hood,  Mt.  Shasta,  Mt.  Rainier  and  others  are 
chiefly  andesite.  The  products  of  Mexican  and  South  American 
volcanoes  are  also  of  this  type,  and  indeed  along  the  whole  Pacific 
border  the  recent  lavas  have  many  features  in  common.  Abroad 
andesites  are  seldom  lacking  in  great  volcanic  districts. 


THE    BASALTS,    INCLUDING    DIABASE. 


Basalts. 

Si02. 

A12O3. 

Fe203. 

FeO. 

CaO. 

MgO. 

Na^O 

.    K2O. 

Loss. 

Sp.  Gr. 

i. 

57.25 

16.45 

167 

i-77 

7.65 

6.74 

3.00 

'•57 

o-45 

"  •  2. 

53-8i 

13.48 

3-02 

7-39 

10.34 

6.46 

3-23 

0.64 

0-57 

2-75 

3- 

53-62 

22.09 

4.21 

.    . 

6.O2 

6.24 

3-i6 

o-57 

5-°3 

4. 

52.27 

17.68 

2.51 

5.00 

8.39 

6.05 

4.19 

1.58 

0.82 

5- 

51.58 

11.92 

2.96 

13.05 

8.52 

4.09 

0-95 

0.34 

1.52 

2.989 

6. 

50.38 

19.83 

6.05 

2.00 

10.03 

5-36 

2.15 

1.76 

J-37 

—  7. 

49-45  i 

17.58 

3.41 

3-41 

7.20 

4.05 

5.83 

J-57 

4-34 

8. 

49.04 

i8.it 

2.71 

7.70 

7.11 

4.72 

4.22 

2.  1  1 

1.29 

2.738 

9- 

48.40 

17-95 

2.28 

8.85 

10.05 

6.99 

2.86 

1.03 

0-34 

2.8 

10. 

47-54 

19.52 

4.24 

6.95 

11.70 

6.66 

3-09 

0.16 

2.981 

IT. 

46.43 

17.10 

ii.  16 

.     . 

10.38 

9.78 

2.50 

.   . 

2.65 

Diabases 

12. 

54.52 

19.10 

2.83 

5.89 

7-25 

3-92 

3-73 

2.30. 

0-59 

2.7 

13- 

53-13 

13-74 

1.08 

9.IO 

9-47 

8.58 

2.30 

1.03 

0.90 

2.96 

14. 

49.28 

15.92 

1.91 

10.20 

7-44 

5-99 

3-40 

0.72 

3-90 

2.86 

I5- 

48.75 

17.17 

0.41 

13.62 

8.82 

3-37 

1.63 

2.40 

.    . 

2.985 

-16. 

46.28 

12.96 

4.67 

6.06 

10.12 

8.71 

3- 

75 

3-34 

2.921 

17- 

45-46 

19.94 

15-36 

.     . 

8.32 

2-95 

2.12 

3-2i 

2.30 

2-945 

Limburgite. 

1  8. 

46.90 

10.17 

1.22 

5-'7 

6.20 

20.98 

1.16 

2.04 

5-42 

2.86 

19- 

40.22 

14.41 

17.42 

2.36 

"•53 

7.29 

3-94 

1.90 

I.IO 

2.89 

Nepheline-basalt. 

20. 

38.35 

9.18 

20.32 

.   . 

11.76 

1378 

2.77 

2.O2 

1.  20 

3-223 

I.  Basalt  with  quartz,  Cinder  Cone,  Calif.,  J.  S.  Diller,  A.  J.  S-,  Jan.,  1887,  p.  49, 
Anal.  Hillebrand.  2.  Kilauea,  Sandwich  Is.;  Cohen.,  Neues  Jahrb.  1880,  II. ,41.  3.  Ice- 
land, Schirlitz.  Tsch.  Mitt.,  1882,  440.  4.  Rio  Grande  Canon,  N.  M.,  J.  P.  Iddings,  A.  J. 
S.,  Sept.,  1888,  220,  Anal.  Eakins.  5.  Dalles,  Oregon,  Lemberg,  Z.  d.  d.  g.  G.  XXXV. 
116.  6.  Richmond  Mtn.,  Eureka  Dist,  Nev...  A,  Hague,  Mono,  XX. ,U.  S.  G.  S., 
264,  Analyst  Whitfield.  7.  Point  Bonita,  Calif.,  F.  L.  Ransome,  Bull.  Geol.  Dept, 
Univ.  Calif.,  I.,  106.  8.  Buffalo  Peaks,  North  Park,  Colo.,  \Voodward,  4Oth  Parallel 


THE  BASALTS.  43 

Surv.,  II. ,  126.  9.  Shoshone  Mesa.,  Nev.,  Woodward,  4oth  Par.  Surv.,  II.,  617.  10. 
Cascade  Mts.,  Oregon,  Jannasch,  Tsch.  Mitth.,  1881,  102.  n.  Glassy  basalt, 
Edgecombe  Island,  near  Sitka,  Alaska,  Lemberg,  Z.  d.  d.  g.  G.,  XXXV.,  570.  12. 
Diabase  Hills,  Nev.,  Woodward,  4Oth  Parallel  Surv.,  I.,  Table  opposite  p.  676.  13. 
Penn.  R.  R.  cut,  Jersey  City,  N.  J.,  G.  W.  Hawes,  A.  J.  S.,  iii.,  IX.,  186.  14.  Lake 
Saltonstall,  Conn.,  Ibid.  15.  Dike  near  Boston,  Mass.,  W.  H.  Hobbs,  Bull.  Mus. 
Comp.  Zool.,  XVI.,  i.  1 6.  Point  Bonita,  Calif.,  F.  L.  Ransome,  Bull.  Geol.  Dept., 
Univ.  Calif.,  I.,  106.  17.  Dike  at  Palmer  Hill,  Ausable  Forks,  N.  Y.,  J.  F.  Kemp, 
Bull.,  107.,  U.  S.  G.  S.  26.  1 8.  Limburgite,  Bozeman,  Mont.,  G.  P.  Merrill,  Proc.,  U. 
S.  Natl.  Mus.,  XVII.,  640,  Anal.  Chatard.  19.  Limburgite,  Palma.,  L.  Van  Werveke* 
Neues  Jahrb.  1879,  485.  20.  Nepheline-basalt,  Pilot  Knob.,  near  Austin,  Texas,  J.  F. 
Kemp,  Amer.  Geol.,  Nov.,  1890,  293. 

Comments. — The  first  analysis  is  very  like  the  more  basic  andesites, 
except  in  its  high  percentage  of  MgO.  It  is  of  a  curious  and  excep- 
tional basalt  with  quartz  phenocrysts,  regarding  which,  mention  is 
made  later.  In  general,  the  others  are  notably  high  in  the  oxides  of 
iron,  in  CaO  and  MgO.  The  specific  gravity  is  also  high.  The  an- 
alyses of  diabases  differ  in  no  essential  from  those  of  true  basalts. 
No.  1 3  is  of  especial  interest,  as  it  is  the  one  usually  quoted  as  the 
representative  of  our  Triassic  diabases.  The  last  three  are  repre- 
sentatives of  the  unusual  varieties,  later  mentioned,  that  are  of  rare 
occurrence  in  the  United  States,  but  that  represent  the  limiting 
percentages  of  SiO2  in  rocks  mostly  composed  of  silicates. 

Mineralogical  Composition.  Varieties. — The  name  basalt  is  a 
a  very  ancient  term  and  has  been  explained  in  several  ways. 
Many  regard  it  as  a  corruption  of  basanites  which  was  used  by 
Pliny,  but  for  what  rock  is  uncertain.  The  Greek  word  for  the 
black  touchstone  or  Lydian  stone  used  by  the  ancient  jewellers  is 
similar  to  this  last  form.  Others  refer  it  to  Basan  or  Bashan,  the 
kingdom  of  Og,  as  mentioned  in  the  Old  Testament,  Deuteronomy 
III.,  i.  Again  an  Ethiopian  word  "basal"  used  by  Pliny  for  an 
iron-bearing  rock,  has  been  suggested.  Agricola  in  the  sixteenth 
century  gave  it  its  present  signification. 

It  stands  for  a  very  large  and  important  group,  which  has  many 
mineralogical  varieties,  but  which  can  seldom  be  subdivided  with- 
out exact  microscopical  study.  The  name  basalt,  therefore,  em- 
braces them  all  when  megascopically  considered.  They  are  all 
heavy,  black,  gray  or  brown  rocks,  usually  porphyritic,  but  at 
times  lacking  all  phenocrysts  and  merely  a  closely  crystalline  dark 
rock.  Plagioclase,  augite,  olivine  and  magnetite  are  the  chief 
minerals  present,  and  the  groundmass  is  usually  a  finely  crystalline 
aggregate  of  these  and  of  some  dark  glass.  At  the  acidic  extreme 


44  A   HANDBOOK  OF  ROCKS. 

of  basalts  are  certain  dark  rocks,  with  abundant  augite,  that  yet 
lack  olivine.  Though  closely  related  to  the  augite-andesites,  they 
are  sometimes  called  olivine-free  basalts.  The  typical  basalt  has, 
however,  olivine,  and  often  exhibits  this  mineral  in  large  rounded 
masses.  Coarsely  crystalline  basalts  are  called  dolerites,  a  very 
common  and  useful  field  term.  Typical  dolerites  are  porphyritic 
but  shade  into  granitoid  varieties.  An  old  group  of  rocks  and  one 
whose  name  often  puzzles  beginners  as  regards  its  special  signifi- 
cance is  called  diabase.  The  diabases  are  generally  entirely 
crystalline  and  apparently  of  a  granitoid  texture.  The  feldspars 
are,  however,  in  long  and  relatively  narrow  crystals,  as  contrasted 
with  the  broader  ones  of  typical  granitoid  rocks.  In  the  inter- 
stices of  these  lath-shaped  feldspars  are  found  the  dark  silicates  and 
magnetite.  The  texture  is  called  ophitic  and  is  more  especially  of 
microscopic  importance.  There  is,  therefore,  ground  for  difference 
of  opinion  as  to  whether  the  diabases  should  be  placed  with  the 
granitoid  or  with  the  porphyritic  rocks,  but  as  they  are  always  in 
sheets  or  dikes,  which  shade  into  porphyritic  forms  on  the  contacts, 
and  which  are  really  volcanic  in  their  nature,  they  are  put  here  as 
essentially  basalts  at  the  extreme  of  the  group  toward  the  granitoid 
division.  In  former  times  the  name  was  only  used  for  pre-Tei  tiary 
rocks ;  but  as  often  stated,  the  time  distinction  has  been  long  since 
exploded.  There  are  both  diabases  and  olivine-diabases,  but  really 
except  in  connection  with  microscopic  work  the  term  diabase  is 
superfluous,  while  we  have  and  use  basalt,  dolerite  and  gabbro.  It 
is,  however,  so  intimately  involved  with  the  literature  of  many  im- 
portant mining  regions  and  others  of  great  geological  interest  that 
the  student  should  be  familiar  with  its  employment  and  its  signifi- 
cance. The  name  is  derived  from  the  Greek  verb  meaning  to. 
penetrate  or  pass  through,  and  was  suggested  by  the  dikes  in  which 
the  early  occurrences  were  met.  Greenstone  and  trap  are  also 
old  names  chiefly  applied  to  diabase. 

Into  many  basalts,  nepheline  or  leucite  enter,  and  if  in  notable 
.amount,  with  little  or  no  olivine,  the  rock  is  called  tephrite,  or 
leucite- tephrite  ;  if  with  much  olivine,  basanite  or  leucite-basanite ; 
if  with  little  or  no  olivine  and  no  plagioclase,  nephelinite  or  leuci- 
tite ;  and  if  there  is  much  olivine,  nepheline-basalt  and  leucite- 
basalt.  The  distinctions  are,  however,  microscopic  and  in  the  field 
basalt  is  sufficient.  Again  the  rocks  may  lack  both  the  feldspar 
and  the  feldspathoids  and  consist  merely  of  augite  in  a  glassy 


THE  BASALTS.  45 

groundmass,  i.  e.,  augitite,  or  of  augite  and  olivine  in  a  glassy  ground- 
mass,  giving  limburgite,  but  these  are  also  only  of  microscopic 
moment,  although  the  significance  of  the  names  should  be  under- 
stood. Basalts  with  melilite  are  the  rarest  of  the  group. 

Basalts  rather  rarely  have  hornblende,  but  when  this  is  present 
it  is  a  deep  brown  variety  known  as  basaltic  hornblende.  Biotite 
is  also  uncommon,  except  in  the  varieties  with  nepheline  or  leucite. 
A  most  extraordinary  basalt  has  been  met  by  J.  S.  Diller  and  others 
in  our  western  volcanic  regions  that  contains  quartz  in  moderately 
large  phenocrysts.  The  presence  of  this  mineral  in  such  basic 
rocks  is  most  peculiar,  but  it  is  explained  by  assuming  exceptional 
conditions  of  crystallization  in  the  early  history  of  the  magma,  lead- 
ing to  the  separation  of  quartz  which  was  never  re-absorbed. 

Alteration,  Metamorphism. — The  olivine  of  basaltic  rocks  is  the 
first  mineral  to  alter,  and  it  soon  becomes  a  network  of  serpentine 
veinlets  enclosing  unchanged  nuclei.  The  augite  also  passes 
readily  into  chlorite  and  finally  the  feldspar  kaolinizes.  The 
prevalence  of  green,  chloritic  products  suggested  the  name  green- 
stone for  the  old  diabases.  The  basaltic  rocks  are  extremely  im- 
portant in  connection  with  metamorphism,  and  the  iron-mining 
regions  around  Lake  Superior  present  superb  illustrations  of  the 
process.  The  augite  has  the  greatest  tendency  to  pass  into  green 
hornblende,  by  what  is  called  a  "  paramorphic  "  change,  t.  e.,  a 
change  in  the  mineral  without  change  in  the  chemical  composition 
and  without,  as  in  pseudomorphs,  preserving  the  original  form. 
Under  shearing  stresses  and  movements,  accompanied  by  this 
paramorphic  change,  diabases,  so-called,  pass  into  hornblende- 
schists,  and  even  chlorite-schists  or  green-schists,  losing  their  mas- 
sive structure  entirely  and  becoming  a  very  different  rock,  and  one 
that  can  be  traced  to  its  original  with  great  difficulty.  Such  horn- 
blendic  rocks  are  also  called  amphibolites. 

Tuffs.  Basaltic  tuffs,  agglomerates,  breccias,  etc.,  are  well  known 
and  often  accompany  the  massive  flows.  They  mark  an  explosive 
stage  of  eruption  before  the  actual  outpouring  of  lava. 

Distribution.  Basaltic  rocks  are  enormously  developed  in  this 
country.  The  oldest  strata  are  penetrated  by  numerous  black, 
igneous  dikes,  in  practically  all  their  exposures.  The  New  Eng- 
land seacoast  is  especially  seamed  by  them,  and  hundreds  may  be 
met  in  a  short  distance.  The  Adirondacks  and  the  White  Moun- 
tains, the  Highlands  of  New  York  and  New  Jersey,  have  many. 


46  A   HANDBOOK  OF  ROCKS. 

In  the  East  are  the  intruded  sheets  of  Triassic  diabase,  up  to  500 
feet  in  thickness,  forming  many  of  the  most  prominent  landmarks, 
such  as  Cape  Blomidon,  N.  S.;  Mts.  Tom  and  Holyoke,  Mass.; 
East  and  West  Rock,  near  New  Haven,  Conn.;  the  Palisades  on 
the  Hudson,  and  many  dikes  in  the  Richmond,  Va.,  and  Deep  River, 
N.  C.,  coal  fields.  Around  Lake  Superior,  both  in  the  iron  and  in 
the  copper  regions,  are  still  greater  sheets,  for  many  thousands  of 
feet  of  basalt  (diabase)  are  present  on  Keweenaw  Point.  On  the 
north  shore  near  Port  Arthur,  the  head-lands  of  Thunder  Bay  ex- 
hibit superb  examples.  The  iron-bearing  strata  are  penetrated  by 
innumerable  dikes.  The  greatest  of  all  the  American  basaltic  areas 
is,  however,  met  in  the  Snake  River  region  of  southern  Idaho  and 
extends  into  eastern  Oregon  and  Washington.  Many  thousands 
of  square  miles  are  covered  with  the  dark  lava  and  are  locally 
called  the  "  Lava  Beds."  In  Colorado,  as  at  the  Table  Mtns.,  near 
Golden,  and  Fisher's  Peak,  near  Trinidad,  there  are  prominent 
sheets,  and  the  same  is  true  of  many  other  points  in  this  State. 
In  New  Mexico,  Arizona  and  Texas  they  are  also  met.  The  vol- 
canoes of  the  Sandwich  Islands  are  basaltic.  Basaltic  rocks  with 
nepheline  are  scarcely  known  in  the  United  States.  Some  minor 
dikes  in  the  East,  a  volcanic  neck  at  Pilot  Knob,  near  Austin, 
Texas,  dikes  and  sheets  in  Uvalde  Co.,  Texas,  and  a  few  dikes  at 
Cripple  Creek,  Colorado,  are  practically  the  only  localities  yet 
identified.  Leucitic  rocks,  more  phonolitic  than  basaltic,  are  known 
in  the  Leucite  Hills,  Wyo.,  and  in  Arkansas.  Of  basaltic  affinities 
they  occur  in  New  Jersey,  but  these  and  the  nepheline  rocks  are 
of  small  practical  moment,  although  of  great  scientific  interest. 

Basalts  have  quite  as  great  development  abroad  as  here.  The 
islands  off  the  north  coast  of  Scotland  are  famous  localities,  and 
many  of  the  volcanic  regions  of  the  continent  are  no  less  well  pro- 
vided. The  lavas  of  Etna  are  chiefly  basaltic,  and  those  of 
Vesuvius  are  remarkable  for  their  richness  in  leucite.  In  India  are 
the  great  basalt  fields  of  the  Deccan,  which  are  comparable  in  ex- 
tent with  those  of  the  Snake  River  region  of  the  West. 


2. 

65-27 

I5-76 

1.36 

3-44 

2.14 

4-57 

3-97 

.    . 

.42 

& 

61-75 

18.88 

0.52 

3-52 

3-54 

1.90 

3.67 

1.24 

4.46 

4- 

58-°5 

18.00 

2.49 

4-56 

6.17 

3-55 

3-64 

2.18 

.86 

5- 

56.71 

18.36 

.   . 

6-45 

6.  1  1 

3-92 

3-52 

2.38 

6. 

52.35 

15-72 

2.90 

7-32 

8.98 

7.36 

2.81 

1.32 

1-35 

CHAPTER   V. 

THE  IGNEOUS  ROCKS,  CONTINUED.     THE  DIORITES,  GABBROS, 

PYROXENITES  AND  PERIDOTITES.      ULTRA-BASIC 

IGNEOUS  ROCKS. 

THE  DIORITES. 

SiO2.      A12O3.      Fe2O3.     FeO.     CaO.      MgO.    Na2O.    K2O.     Loss.  Sp.Gr. 
67.54        17.02         2.97         .34       2.94        1.51       4.62       2.28         .55 

2.79 
2.86 

7.  50.47       18.73         4-19       4-92       8.82       3.48       4.62       3.56         .58      2.87 

8.  48.98        17.76         2.14       6,52       8.36       2.09       6.77       2.08       4.50 

9.  48.19        16.79        18.37        .    .         6.85        1.32       5.59        i.n        2.31 

i.  Quartz-mica-diorite,  Electric  Peak,  Yellowstone  Park,  J.  P.  Iddings,  Anal,  by 
Whitfield,  Bull.  Phil.  Soc.  of  Washington,  II,  206.  2.  Quartz-augite-diorite,  Watab, 
Minn.,  A.  Streng,  Neues  Jahrbuch,  1877,  232.  3.  Diorite.  Pen-maen-mawr,  Wales, 
J.  A.  Phillips,  Q.  J.  G.  S.,  XXXIII,  424,  1877.  4.  Diorite,  see  under  No.  i.  5. 
Diorite,  (granitoid  andesite  ?)  Comstock  Lode,  Nev.,  R.  W.  Woodward,  4Oth  Par. 
Survey,  I,  opp.  p.  676.  6.  Augite-diorite,  Little  Falls,  Minn.,  A.  Streng,  Neues  Jahrb., 
1877,  129.  7.  Augite-diorite,  Mt.  Fairview,  Custer  Co.,  Colo.,  W.  Cross,  Anal,  by 
Eakins,  Col.  Sci.  Soc.,  1887,  247.  8.  Porphyritic-diorite,  St.  John,  N.  B.,  W.  D. 
Matthew,  Trans.  N.  Y.  Acad.  Sci.,  XIV,  213.  9.  Diorite  dike  rich  in  magnetite,  Forest 
of  Dean  Mine,  N.  Y.,  J.  F.  Kemp,  A.  J.  S.,  Apr.,  1888,  331. 

Comments. — The  analyses  represent  a  series  that  is  closely  parallel 
with  the  dacites  and  andesites  in  the  first  five  analyses,  but  that 
recalls  the  basalts  and  diabases  in  the  last  four.  MgO  rules  lower 
than  in  the  latter.  It  is  evident  that  the  prevailing  feldspar  would 
be  a  lime-soda  variety,  but  that  orthoclase  might  readily  be  pro- 
duced, for  even  when  we  allow  some  for  biotite  the  percentage  of 
K2O  is  notable.  The  parallelism  with  gabbros  will  be  shown  by 
the  next  table  to  be  close. 

Miner alogical  Composition  and  Varieties. — The  name   diorite   is 

47 


48  A   HANDBOOK  OF  ROCKS. 

derived  from  the  Greek  verb,  meaning  to  distinguish  in  allusion  to 
the  fact  that  the  hornblende  and  feldspar  could  be  distinguished  one 
from  another  in  the  coarsely  crystalline'ones  first  studied.  Diorites 
are  granitoid  rocks  consisting  of  hornblende,  biotite  and  plagio- 
clase.  Those  with  hornblende  are  diorites  proper,  while  those 
with  biotite  are  called  mica-diorites.  Some  augite  is  occasionally 
present,  marking  passages  to  the  gabbros  and  giving  the  name 
augite-diorite.  An  intermediate  type  occurring  in  dikes,  and  con- 
taining both  biotite  and  augite  is  kersantite.  Acidic  diorites  often 
have  considerable  quartz,  and  are  called  quartz-diorites.  Tonalite  is  a 
quartz^hornblende-diorite.  Quartz-diorites  shade  insensibly  into 
granites,  and  the  importance  of  the  intermediate  forms  or  granite- 
diorites  was  emphasized  under  granite.  These  acidic  diorites  are 
prevailingly  light  in  color,  but  the  more  basic  ones  become  de- 
cidedly dark.  Certain  dikes  with  the  minerals  of  diorite  are  called 
camptonites.  The  great  tendency  of  augite  to  change  to  green 
hornblende  causes  a  doubt  to  hang  over  the  true  character  of 
many  diorites.  They  may  often  be  metamorphic  products  from 
gabbros  or  diabases. 

Alteration,  Metamorphism. — In  ordinary  alteration  the  feldspar  of 
diorites  kaolinizes  and  the  hornblende  changes  to  chlorite,  affording 
one  of  the  varieties  of  the  so-called  greenstones.  In  metamorphism 
the  diorites  pass  into  gneisses,  under  shearing  stresses,  and  into 
hornblende  schists  or  amphibolites.  In  many  mining  regions  even 
decidedly  schistose  varieties  are  still  called  diorite.  A  final  stage 
is  chlorite-schist,  wherein  the  hornblende  has  altered  to  chlorite. 

Distribution.  True,  original  diorites  are  not  very  common  rocks 
in  America.  A  well  known  quartz-mica-diorite  is  extensively  de- 
veloped in  a  series  of  igneous  rocks,  called  the  Cortlandt  Series, 
on  both  sides  of  the  Hudson,  below  Peekskill.  In  the  Sudbury 
nickel  district,  north  of  Lake  Huron,  dense,  dark  diorites  are  the 
chief  rock  containing  the  ore,  but  there  is  always  the  possibility 
that  the  hornblende  is  altered  augite.  Mt.  Davidson,  above  the 
Comstock  Lode,  is  either  a  true  diorite  or  a  granitoid  phase  of 
andesite.  Authorities  differ  as  to  its  interpretation.  Grano- 
diorites,  the  intermediate  rocks  between  granite  and  diorite,  are 
well  recognized  both  in  the  East  and  the  West. 

Diorites  are  well  known  abroad  and  have  been  described  from 
various  places  in  Great  Britain,  Germany,  France  and  Austria. 
The  typical  tonalite  is  obtained  near  Meran,  in  the  Tyrol, 


GAB  BROS,  PYROXEN1TES  AND  PERIDOTITES.      49 


THE    GABBROS,    PYROXENITES   AND    PERIDOTITES. 


Gabbro. 

SiO2. 

A1203. 

Fe203. 

FeO. 

CaO. 

MgO. 

Na20. 

K20. 

Loss. 

Sp.  Gr. 

i. 

59-55 

25.62 

0.75 

.    . 

7-73 

tr. 

5-°9 

0.96 

o.45 

2.66 

2. 

55-34 

1637 

0.77 

7-54 

7-51 

5-°5 

4.06 

2.03 

0.58 

3-  f 

54.72 

17.79 

2.08 

6.03 

6.84 

5-85 

3.02 

3.01 

.    . 

2.928 

4- 

54-47 

26.45 

1.30 

0.67 

10.80 

0.69 

4.37 

0.92 

0-53 

2.72 

5- 

53-43 

2801 

0-75 

.    . 

11.24 

0.63 

4-85 

0.96 

tr. 

2.67 

6. 

52.14 

29-17 

3-26 

10.81 

0.76 

3.02 

0.98 

0.58 

•7  7. 

49.15 

21.90 

660 

4-54 

8.22 

3-°3 

3.83 

1.61 

1.92 

8. 

48.02 

17-S° 

i.  80 

7.83 

I3.I6 

IO.2I 

1.48 

tr. 

•79 

9- 

46.85 

19.72 

3.22 

7-99 

I3.IO 

7-75 

1.56 

0.09 

.56 

10. 

46.85 

18.00 

6.16 

8.76 

10.17 

8.43 

2  19 

0.09 

•  30 

3-097 

ii. 

45.66 

16.44 

0.66 

13.90 

7.23 

"•57 

2.13 

.41 

.07 

Pyroxenite. 

12. 

55-  J4 

0.25 

3-48 

4-73 

8-39 

26.66 

•30 

. 

IS- 

53-98 

1.32 

1.41 

390 

15-47 

22.59 

.     . 

.    . 

0.83 

3-3°i 

J4- 

44  01 

11.76 

15.01 

4.06 

25.25 

Peridotite. 

I5- 

47.41 

6.39 

7.06 

4.80 

H.32 

15.34 

.69 

1.40 

2.10 

3.30 

1  6. 

46.03 

9.27 

2.72 

9-94 

3-53 

25.04 

1.48 

0.87 

0.64 

3.228 

17. 

4I.OO 

7.58 

.    . 

5-99 

10.08 

23-59 

0.52 

. 

4-73 

2.989 

1  8. 

36.80 

4?i6 

8.33 

8.63 

25.98 

O.I7 

2.48 

0.51 

19. 

33-84 

5.88 

7-°4 

5.16 

9.46 

22.96 

0-33 

2.04 

75° 

20. 

29.81 

2.01 

5.16 

4-35 

7.69 

32.41 

O.I  I 

O.2O 

8.92 

2.78 

I.  Anorthosite,  Chateau  Richer,  Canada,  T.  S.  Hunt,  Geology  of  Canada,  1863.  2. 
Norite,  Cortland  Series,  Montrose  Point,  Hudson  River,  Anals.  by  Munn,  for  J.  D.  Dana, 
A.  J.  S.,  Aug.,  1881,  p.  104.  3.  Gabbro,  near  Cornell  Dam,  Croton  River,  H.  T.  Vulte, 
for  J.  F.  Kemp,  unpublished.  4.  Anorthosite,  Summit  of  Mt.  Marcy,  Adirondacks,  A.  R. 
Leeds,  3Oth  Ann.  Rep.,N.  Y.  State  Museum,  reprint,  p.  14,1876.  5.  Anorthosite,  Nain, 
Labrador,  A.  Wichmann,  Z.  d.  d.  g.  Ges.,  1884,  6.  Gabbro,  Iron  Mtn.,  Wyo., 
4Oth  Parallel  Surv.,  II.,  14.  7.  Gabbro,  near  Duluth,  Minn.,  Streng,  Neues  Jahrb. 
1876,  117.  8.  Gabbro-diorite,  Baltimore,  Md.,  average  of  seventeen  samples,  Mackay 
for  G.  H.  Williams,  U.  S.  G.  S.,  Bull.,  XXVIII.,  37.  9.  Gabbro,  Baltimore 
average  of  twenty-three  samples,  ibid.  10.  Gabbro,  Southwest  Adirondacks, 
C.  H.  Smyth,  Jr.,  A.  J.  S.,  July,  1894,  61.  n.  Gabbro,  Northwest  Minn.,  W. 
S.  Bayley,  Anals.  by  Stokes,  Jour.  Geol.  I.,  712.  12.  Pyroxenite,  var.  Web- 
sterite,  Webster,  N.  C.,  E.  A.  Schneider  for  Geo.  H.  Williams,  Amer.  Geol.,  July, 
1890,  p.  41.  13.  Pyroxenite,  Baltimore,  Md.,  T.  M.  Chatard  for  G.  H.  Williams, 
ibid.  14.  Pyroxenite,  Meadow  Creek,  Mont.,  Geo.  P.  Merrill,  Proc.,  U.  S.,  Nat'l 
Mus.,  XVII.",  658.  15.  Peridotite,  Cortland  Series,  Montrose  Pt.,  N.  Y.,  Emerson 
for  G.  H.  Willams,  A.  J.  S.,  Jan.,  1886,  40.  16.  Peridotite,  Custer  Co.  Colo.,  L.  G. 
Eakins  for  W.  Cross,  Proc.  Colo.  Sci.  Soc.,  1887,  245.  17.  Peridotite,  Baltimore, 
Md.,  L.  Mackay  for  G.  H.  Williams,  Amer.  Geol.,  July,  1890,  39.  1 8.  Peridotite, 
Dewitt,  N.  Y.,  H.  S.  Stokes  for  Darton  and  Kemp,  Amer.  Jour.  Sci.,  June,  1895,  456- 
19.  Mica  Peridotite,  Crittenden  Co.,  Ky.,  W.  F.  Hillebrand  for  J.  S.  Diller,  A.  J.  S., 
Oct.,  1892,  288.  20.  Peridotite,  Elliott  Co.,  Ky.,  J.  S.  Diller,  Bull.  38,  U.  S.  G.  S., 
p,  24. 


50  A   HANDBOOK  OF  ROCKS. 

Comments. — The  range  in  composition  presented  by  the  gabbros 
is  in  many  respects  the  same  as  that  of  the  basalts.  As  a  general 
rule  the  most  feldspathic  members  (the  anorthosites)  are  the 
highest  in  silica,  Nos.  I,  4  and  5.  No.  6,  although  described  as 
gabbro,  is  doubtless  of  the  same  character,  for  the  low  FeO  and 
MgO  indicate  few  dark  silicates.  These  rocks  are  also  highest  in 
A12O3  of  all  the  rock  analyses  yet  quoted.  As  the  CaO  and  MgO 
increase  in  amount,  the  pyroxenes  and  olivine  grow  notably  more 
abundant.  In  the  gabbros  of  the  Cortland  series  ( Nos.  2  and  3  )  and 
in  those  near  Duluth  (No.  7)  there  is  often  considerable  orthoclase 
as  is  indicated  by  the  K2O.  The  pyroxenites  are  distinguished  by 
the  falling  off  in  A12O3,  due  to  the  disappearance  of  feldspar,  and 
by  the  increase  in  CaO  and  MgO  from  the  pyroxenes.  The  perido- 
tites  reach  a  lower  percentage  of  silica  than  any  other  igneous 
rocks  so  far  cited,  but  if  this  is  accompanied  by  high  H2O,  allow- 
ance must  be  made  for  the  relative  decrease  of  the  original  SiOz  in 
the  change  to  serpentine.  The  great  percentages  of  MgO  are  very 
notable,  and  are  due  to  the  presence  of  much  olivine,  magnesian 
pyroxene  and,  in  instances,  biotite.  Chromic  oxide  is  also  always 
present  in  small  amounts,  and  oxides  of  nickel  and  cobalt  are 
usually  in  perceptible  quantity. 

Mineralogical  Composition,  Varieties. — The  name  gabbro  is  of 
Italian  origin,  and  has  been  applied  of  recent  years,  and  with 
growing  favor  to  the  great  group  of  granitoid  rocks  consisting  in 
the  typical  cases  of  plagioclase  and  pyroxene.  The  diabases,  as 
explained  under  basalt,  are  now  generally  classed  with  the  vol- 
canic rocks,  although  texturally  and  mineralogically  they  really 
lap  over  true  gabbros.  The  so-called  gabbro  group  is  a  very  large 
and  characteristically  variable  one.  Originally  the  name  gabbro 
was  only  applied  to  a  mixture  of  plagioclase  and  the  variety  of 
monoclinic  pyroxene  called  diallage,  that  has  pinacoidal  as  well 
as  prismatic  cleavages,  but  of  late  years  all  granitoid,  plutonic 
pyroxene-plagioclase  rocks  are  collectively  spoken  of  as  the 
gabbro  group.  At  the  acidic  extreme  we  have  in  Canada  and 
the  Adirondacks  enormous  masses  of  rock  that  are  practically  pure, 
coarsely  crystalline  labradorite.  Pyroxene  is  the  dark  silicate 
when  any  is  present,  but  often  it  is  insignificant.  These  pure  feld- 
spar rocks  are  best  called  anorthosites,  from  the  French  word  for  tri- 
clinic  feldspar,  but  the  word  is  not  to  be  confused  with  anorthite, 
the  lime-feldspar  with  which,  it  has  no  special  connection.  As 


GAB  BROS,  PYROXENITES  AND  PERIDOTITES.      51 

monoclinic  pyroxene  increases  they  pass  into  gabbros  proper. 
More  or  less  biotite  and  hornblende  may  also  be  present.  If  the 
pyroxene  is  orthorhombic  we  call  the  rock  norite.  Varieties  with 
olivine  are  frequent,  giving  olivine-gabbro  and  olivine-norite. 
Gabbros  and  norites  are  not  readily  distinguished  without  the 
microscope,  unless  the  bronzy  appearance  of  hypersthene  can  be 
recognized.  Gabbro  is  then  a  good  collective  term.  An  old  and 
obsolete  synonym  of  anorthosite  is  labradorite-rock,  of  interest 
because  widely  used  in  the  early  reports  on  the  Adirondacks. 
Norites  were  called  hypersthene  rock,  or  hypersthene-feis,  both  of 
which  are  undesirable  rock  names.  Gabbro  intrusions  of  not  too 
great  extent  or  irregularity  for  careful  study  have  been  observed 
to  grow  more  basic  toward  the  outer  margins. 

The  gabbros  pass  insensibly,  by  the  decrease  of  plagioclase,  into 
the  pyroxenites  and  peridotites,  and  in  any  great  gabbro  area  all 
these  are  usually  present,  but  they  may  occur  also  as  independent 
masses.  The  pyroxenites  are  practically  pyroxene,  with  little  if 
any  other  minerals.  There  is  some  variety,  according  as  the  rock 
contains  one  or  several  of  the  following :  enstatite,  bronzite,  hypers- 
thene, diallage  or  augite;  but  with  the  unassisted  eye,  it  is  seldom 
that  one  can  be  sure  of  these  distinctions,  except  as  the  ortho- 
rhombic  pyroxenes  exhibit  a  bronze  luster.  Hornblende,  magnetite 
and  pyrrhotite  may  also  be  present.  With  the  accession  of  olivine, 
peridotite  results,  so  named  from  the  French  word  peridot  for 
olivine,  and  a  number  of  varieties  have  been  made  according  as  the 
olivine  is  associated  with  one  or  more  of  the  minerals  cited  for 
pyroxenites.  The  distinctions  are  however  hardly  possible  with- 
out microscopic  aid.  As  the  extreme  of  peridotites  we  have  a 
nearly  pure  olivine  rock,  called  dunite,  important  in  North  Caro- 
lina. Much  magnetite  may  be  associated  with  peridotite ;  indeed 
at  Cumberland  Hill,  R.  I.,  there  is  enough  to  almost  make  the  rock 
an  ore,  Chromite,  too,  is  a  frequent  associate.  As  peridotites 
shade  into  a  porphyritic  texture,  especially  in  dikes,  they  have  been 
called  picrites,  and  even  further  varieties,  such  as  kimberlg^te, 
have  been  made.  Black  hornblende,  which  is  brown  in  thin 
sections,  is  frequent  in  both  pyroxenites  and  peridotites,  and  may 
even  form  a  rock  itself,  hornblendite.  Dark  brown  biotite  is  also 
often  present  in  considerable  amount. 

Some  writers  have  regarded  the  pyroxenites  and  peridotites  as 
of  doubtful  igneous  origin  and  have  placed  them  with  metamorphic 


52  A   HANDBOOK  OF  ROCKS. 

rocks,  but  from  their  frequent  association  with  gabbro,  and  from 
their  independent  occurrence  in  dikes,  there  is  no  good  reason  to 
doubt  their  true,  igneous  nature. 

A  very  rare  granitoid  rock,  consisting  of  plagioclase,  nepheline 
and  ferro-magnesian  silicates  has  been  called  theralite  from  the 
Greek  verb  to  seek  eagerly,  because  its  discovery  was  anticipated 
before  it  was  actually  found  by  J.  E.  Wolff  in  the  Crazy  Mountains, 
Montana.  It  is  an  extremely  rare  combination  of  minerals,  but  of 
special  scientific  interest  because  it  corresponds  among  the  gran- 
itoid rocks  to  the  tephrites  and  basanites  of  the  porphyritic. 

Alteration,  Metamorphism. — The  gabbros  alter  chiefly  by  the 
formation  of  serpentine  and  chlorite  from  the  dark  silicates.  The 
pyroxenites  and  peridotites  change  readily  into  serpentine, 
often  with  an  intermediate  stage  as  hornblende-schist.  Under 
dynamic  stresses,  especially  shearing,  anorthosites  and  gabbros 
pass  into  gneissoid  types,  and  in  the  process  much  garnet  may  be 
developed.  This  is  especially  true  in  the  Adirondacks.  The 
larger  feldspars  may  be  left  in  the  gneisses  as  "eyes,"  or  to  adopt 
the  German  term,  as  "Augen,"  affording  Augen-gneiss,  i.  e., 
gneisses  with  comparatively  large  lenticular  feldspars.  Much 
hornblende,  especially  in  true  gabbros,  is  often  developed  in  the 
process.  The  basic  members,  the  pyroxenites  and  peridotites 
develop  amphibolites  or  hornblende-schists,  which  latter  often  fur- 
nish very  puzzling  geological  problems. 

Distribution. — The  anorthosites  are  so  far  as  we  know  limited  to 
several  Canadian  areas,  as  at  the  headwaters  of  the  Saguenay  river, 
and  again  north  of  Montreal;  and  to  the  higher  peaks  of  the 
Adirondacks  and  some  of  their  outliers.  Mt.  Marcy  and  its 
neighbors  consist  of  them.  Gabbros  are  also  present  in  vast  quan- 
tity, and  are  likewise  well  known  in  the  White  Mountains,  in  the 
famous  Cortlandt  series,  near  Peekskill,  on  the  Hudson,  and  in  the 
vicinity  of  Baltimore.  Around  Lake  Superior  gabbros  are  of 
great  importance,  as  the  basal  members  of  the  Keweenawan  system 
and  other  older  intrusions  are  largely  formed  of  them.  Fine 
specimens  can  be  had  at  Duluth.  They  are  a  characteristic  wall 
rock  of  titaniferous  magnetite.  Pyroxenites  occur  as  subordinate 
members  of  the  gabbro  areas,  especially  near  Baltimore.  Peridotites 
are  in  the  same  relations  in  the  Cortlandt  series,  in  the  Baltimore 
area  and  in  North  Carolina.  They  are  also  known  on  Little  Deer 
Island,  Me.;  at  Cumberland  Hill,  R.  I.;  in  the  dikes  near  Syracuse, 


GAB  BROS,  PYROXENITES  AND  PERWOTITES.      53 

N.  Y. ;  at  Presqu'  Isle,  near  Marquette,  Mich. ;  in  Kentucky;  in 
California  and  elsewhere  in  the  West.  When  outlying  dikes  are 
met,  far  from  any  visible,  parent  mass  of  igneous  rock  and  in  sedi- 
mentary walls,  they  are  very  frequently  peridotite. 

Abroad,  anorthosites  and  gabbros  are  abundant  in  the  Scandi- 
navian peninsula,  whose  geology  is  in  many  respects  like  that  of 
Canada  and  the  Adirondacks.  In  the  north  of  Scotland  gabbros 
are  of  especial  interest  because  they  have  been  shown  by  Judd  to 
be  the  deep-seated  representives  of  the  surface  basalts.  On  the 
continent  they  are  important  rocks  in  many  localities.  The  same 
is  true  of  Australia  and  such  other  parts  of  the  world  as  have  been 
studied.  Of  especial  interest  are  the  peridotite  dikes  in  South 
Africa  that  have  proved  to  be  the  matrix  of  the  diamond. 

ULTRA-BASIC   IGNEOUS   ROCKS.      METEORITES. 

A  few  ultra  basic  igneous  rocks  are  known  in  which  the  silica 
decreases  almost  to  nil,  and  in  which  the  bases,  especially  iron,  are 
correspondingly  high.  They  are  in  general  rather  to  be  con- 
sidered as  basic  segregations  in  a  cooling  and  crystallizing  magma, 
than  as  individual  intrusions.  The  Cumberland  Hill,  R.  I.,  so- 
called  peridotite,  cited  above,  has  very  little  silica.  Titaniferous 
ores  have  almost  none,  but  they  are  often  exceptionally  rich  in 
alumina.  In  a  few  cases  metallic  iron  has  been  detected  in  basic 
igneous  rocks,  suggesting  analogies  with  meteorites. 

Meteorites  are  rare  and  only  of  scientific  interest,  but  it  is 
extremely  suggestive  that  such  silicates  as  are  met  in  them  are 
chiefly  olivine  and  enstatite,  minerals  rather  characteristic  of  very 
basic  rocks.  The  commoner  meteorites  are  an  alloy  of  metallic 
iron  and  nickel,  but  some  rare  sulphides  are  occasionally  present. 

As  filling  out  the  theoretical  series  we  cannot  bar  out  water  and 
ice.  There  is  no  reason  why  they  are  not  to  be  considered  igneous 
rocks  of  extremely  low  fusing  point,  but  they  are  so  familiar  that 
a  simple  reference  to  them  is  sufficient. 


CHAPTER    VI. 
REMARKS  IN  REVIEW  OF  THE  IGNEOUS  ROCKS. 

Chemical  Composition.  Igneous  magmas  vary  from  about  80% 
silica  as  a  maximum  to  practically  none  as  a  minimum,  but  impor- 
tant rocks  rarely  drop  below  40  .  Alumina  is  highest  in  the 
anorthosites  or  feldspathic  gabbros,  where  it  may  exceed  25%.  It 
is  lowest  in  the  pyroxenites  and  may  be  less  than  i%.  The 
oxides  of  iron  are  almost  lacking  in  the  highly  siliceous,  but  may 
reach  beyond  20%  in  the  basaltic  rocks,  and  with  TiO2  may  be  nearly 
1 00%  in  some  igneous  iron  ores.  Lime  attains  its  maximum  of 
12-15%  in  the  gabbros  and  pyroxenites,  while  magnesia  in  the 
pyroxenites  and  peridotites  may  even  surpass  25  %.  Potash  is  most 
abundant  in  the  granites  and  leucite  rocks;  soda  in  those  with 
nepheline.  Combined  alkalies  may  reach  15%  in  the  phonolites 
and  nepheline-syenites.  In  general  they  are,  however,  about  4-10, 
and  may  practically  fail.  Water  in  quantities  over  I  %  as  a  rule  is 
an  indication  of  decay,  but  in  the  pitchstones  this  is  not  positively 
true,  for  the  water  reaches  close  to  10%  and  the  rocks  .re  appar- 
ently unaltered. 

Texture.  All  three  of  the  typical  textures  are  easily  recognized  in 
characteristic  development,  but  the  glassy  shade  insensibly  into  the 
felsitic,  the  felsitic  into  the  porphyritic,  and  the  porphyritic  into  the 
granitoid.  There  are,  therefore,  intermediate  forms  that  are  diffi- 
cult to  classify.  Yet,  on  the  whole,  the  four  textures  are  the  most 
satisfactory  basis  for  classification,  and  as  a  guide,  in  accordance 
with  which  to  study.  Chemical  composition  being  the  same,  texture 
is  a  result  of  the  physical  conditions  surrounding  the  magma  at  the 
time  of  crystallization  and  of  the  presence  of  mineralizers. 

54 


REVIEW  OF  IGNEOUS  ROCKS. 


55 


RMYOUTES  AND  GRANITES 


RARE  BASIC 
•'s  ORTMOCLASC 
a^.  .  ..  _.  jif  ROCKS 


)ACITES     ANO£SITES  BASALT     GROUP 

,».»  JiORiTCS          O'ORITES        G^B8RO«    P^«ox  CN  i  res   PERIDOT)  TtS  0«C8 

SxQ|y .  .  .  .  y  .  .  .  .  &o .  .  .  .  j-j 


Diagrams  intended  to  illustrate  graphically  the  mineralogical  composition  of  the 
igneous  rocks.  The  numbers  indicate  percentages  in  silica.  The  upper  diagram  in- 
cludes the  orthoclase  rocks,  the  lower  the  plagioclase  and  non-feldspathic  ones.  Q  is 
quartz ;  O,  orthoclase ;  NL,  nepheline  and  leucite  ;  P,  plagioclase ;  M,  muscovite ;  B, 
biotite;  H,  hornblende :  A,  augite;  Ol,  olivine;  the  black  area,  magnetite,  pyrrho- 
tite,  and  other  metallic  minerals. 

Mineralogy.  The  above  diagrams,  with  a  reasonable  approxi- 
mation to  the  truth,  illustrate  the  quantitative  mineralogy  of  the 
igneous  rocks.  A  section  cut  through  the  charts  at  any  one  point 
expresses  the  relative  amounts  as  well  as  kinds  of  the  several  min- 
erals in  the  rocks  whose  names  are  along  the  top  lines,  and  whose 
percentages  in  silica  are  approximately  shown.  No  mention  is 
made  of  texture.  In  the  orthoclase  rocks  quartz  disappears  at 
about  65%  SiO2,  while  orthoclase  continues  to  the  end;  plagio- 
clase in  small  amount  is  quite  constantly  present  throughout  the 
series.  'Nepheline  and  leucite  come  in  as  indicated.  Muscovite 
appears  only  in  the  more  acidic  granites.  Biotite  and  hornblende 
vary  in  relative  quantity,  but  toward  the  basic  end  both  yield  to 
augite.  The  rocks  at  the  basic  end  are  chiefly  those  recently  dis- 
covered by  Weed  and  Pirsson  in  Montana,  by  Iddings  in  the 
Yellowstone  Park  and  by  Lawson  in  the  Rainy  Lake  region.  In 
the  plagioclase  and  non-feldspathic  rocks  quartz  and  orthoclase  soon 
run  out,  so  far  as  any  notable  or  regular  amount  is  concerned. 
Plagioclase  holds  along  to  about  45%  SiO2,  and  at  about  55%  SiO2 
may  in  the  anorthosites  be  the  only  mineral  present.  Biotite  and 


. 
56  A   HANDBOOK  OF  ROCKS. 

hornblende  are  present  all  the  way  through,  but  toward  the 
basic  end  they  tend  to  yield  in  importance  to  augite,  which 
latter  in  some  pyroxenites,  at  about  49%  $iO2,  may  be  the 
only  silicate  present.  Olivine  begins  to  appear  at  55%  and 
steadily  increases  with  occasional  lapses  almost  to  the  end,  where 
it  may  be  the  chief  mineral.  The  ores,  as  the  last  extreme,  and 
without  regard  to  silica,  increase  so  as  to  be  the  only  minerals  in 
the  rock,  forming  thus  the  theoretical  basic  extreme.  The  dia- 
grams also  emphasize  the  fact  that  igneous  rocks  shade  into  one 
another  by  imperceptible  gradations,  and  this  is  true  of  the  ortho- 
clase  and  plagioclase  groups  themselves,  although  not  suggested  by 
the  separation  of  the  two  in  the  drawings.  The  continuation  of  the 
orthoclase  series  to  a  basic  extreme  is  a  fact  that  we  have  only  ap- 
preciated in  very  recent  years. 

A  careful  scrutiny  of  analyses  and  mineralogical  composition 
leads  to  the  conclusion  that  practically  the  same  magma  may, 
under  different  physical  conditions  of  crystallization,  afford  mineral- 
ogical aggregates  that  vary  considerably  in  the  proportions  of  the 
several  minerals — now  yielding  more  hornblende ,  again  more  augite, 
and  even  affording  quartz  in  a  basalt.  Hence,  analyses  in  dif- 
ferent groups  overlap  more  or  less,  and  the  difficulty  of  drawing 
sharp  lines  of  distinction  is  increased.  Yet,  allowing  for  this  vari- 
ation, chemical  composition  determines  the  resulting  mineralogical 
aggregate  and  is  fairly  characteristic. 

Determination  of  Igneous  Rocks.  In  determining  an  igneous 
rock,  the  texture  should  first  be  regarded,  next  the  feldspars.  If 
orthoclase  prevails,  the  presence  or  absence  of  quartz  establishes 
the  rock.  If  plagioclase  prevails,  we  look  for  biotite,  hornblende, 
pyroxene  and  olivine.  If  no  feldspar  is  present  we  look  for  the 
presence  or  absence  of  olivine.  On  this  basis  the  table  on  page 
is  to  used — there  are,  however,  many  finely  crystalline  rocks 
which  elude  the  power  of  the  unassisted  eye.  If  of  light  shades, 
they  can  generally  be  referred  with  reasonable  correctness  to  the 
rhyolites,  trachytes  or  felsites.  If  dark,  the  name  "  trap  "  is  a  very 
useful  and  sufficiently  non-committal  term.  While  books  are  of 
great  assistance,  really  the  only  way  to  become  properly  familiar 
with  rocks,  is  to  use  the  books  in  connection  with  correctly  labeled 
and  sufficiently  complete  study  collections. 

Field  Observations.  A  rock  is  not  to  be  considered  by  any  in- 
telligent observer  as  a  dead  inert  mass  in  nature,  but  as  an  important 


REVIEW  OF  IGNEOUS  ROCKS.  57 

participant  in  the  ceaseless  round  of  changes  that  confront  us  on 
every  side.  Familiarity  with  specimens  and  varieties  in  collections 
ought  always  to  be  followed  by  observation  in  the  field.  We  have 
all  grown  to  believe  that  in  limited  areas  igneous  rocks,  however, 
varied,  they  may  be,  are  yet  intimately  related  in  their  origin,  or 
are  bound  together  by  ties  of  kinship,  "  consanguinity  "  as  Iddings 
has  called  it.  Some  regions  like  eastern  Montana  and  the  Black 
Hills  have  especial  richness  df  high  soda  or  potash  magmas,  giv- 
ing rise  to  nepheline  and  leucite  rocks,  and  sodalite  syenites ;  Colo- 
rado, Utah  and  New  Mexico  have  wonderful  and  enormous  lac- 
colites  of  andesities  (porphyrites).  The  Pacific  coast  in  South 
America  has  andesites  in  vast  extent  from  active  volcanoes,  and  in 
North  America  from  extinct  cones.  Idaho,  Oregon  and  Washing- 
ton are  marked  by  basalts.  The  Atlantic  coast  region  has  a  long 
series  of  very  ancient  volcanoes,  that  preceded  the  early  fossiliferous 
strata  from  Newfoundland  to  North  Carolina  and  that  yielded  nearly 
the  entire  series  of  the  volcanic  rocks.  In  the  Adirondacks,  on  the 
Hudson  near  Peekskill,  near  Baltimore  and  around  Lake  Superior 
we  find  the  members  of  the  gabbro  family ;  while  near  tidewater  along 
the  Atlantic  seaboard  we  have  granites,  almost  all  with  biotite. 
Such  facts  as  these  suggested  the  creation  of  the  term  "petrographic 
provinces,"  to  J.  W.  Judd,  in  the  endeavor  to  suggest  these  kin- 
ships of  magmas  in  certain  limited  districts.  There  are  many 
others  even  in  North  America  that  could  be  cited,  but  the  above  will 
suffice  to  remind  the  reader  that  these  broader  relationships  should 
be  always  before  him  while  extending  his  acquaintance  with  rocks 
as  they  occur  in  the  natural  world  about  him. 


CHAPTER.  VII. 

THE  AQUEOUS  AND  EOLIAN  ROCKS.     INTRODUCTION.     THE 
BRECCIAS  AND  MECHANICAL  SEDIMENTS  NOT  LIMESTONES. 

The  members  of  this,  the  second  grand  division,  are  much 
simpler,  and,  as  a  general  thing,  much  easier  to  identify  and  to 
understand  than  are  the  igneous.  No  single  term  is  comprehensive 
enough  to  include  them  all,  and  even  the  double  one  selected 
above,  in  the  endeavor  to  embrace  as  many  as  possible,  and  to  avoid 
the  multiplication  of  grand  divisions,  still  falls  short  of  including  sev- 
eral. Nevertheless,  those  not  embraced  (the  breccias)  are  of  limited 
distribution,  and,  for  many  reasons,  go  best  with  the  other  fragmental 
rocks,  even  if,  strictly  speaking,  they  are  neither  aqueous  nor  eolian 
in  origin.  Sedimentary  is  a  most  useful  term,  and  is  universally 
applied  as  a  partial  synonym  of  the  above,  for  it  fairly  includes  the 
most  important  members  of  the  series,  but  the  rocks  deposited  from 
solution  and  the  eolian  rocks  can  hardly  be  understood  by  it. 

The  rocks  will  be  taken  up  under  the  following  groups : 

I.  Breccias  and  Mechanical  Sediments,  not  Limestones. 
II.  Limestones. 

III.  Organic  Remains,  not  Limestones. 

IV.  Precipitates  from  Solution. 

The  limestones  are  reserved  for  a  special  group,  although  they 
belong  in  instances  to  each  of  the  other  three.  They  form,  how- 
ever, such  an  important  series  in  their  scientific  and  practical  rela- 
tions, that  it  is  in  many  respects  advantageous  to  take  them  all  up 
together. 

I.     BRECCIAS  AND  MECHANICAL  SEDIMENTS,  NOT  LIMESTONES. 

Group  I.  is  described  in  order  from  coarse  to  fine  in  the  following 
series,  minor  varieties  not  cited  in  the  table  being  mentioned  in  the 
text  under  their  nearest  relatives. 

58 


BRECCIAS. 


59 


COARSE 

TO 

FINE. 

o 

LAND 
ERATE. 

|| 

ARGILLACEOUS 
SANDSTONE. 

SILT  AND 
SHALE. 

CLAY. 

s 

erf 

£S 

ig 

S  | 

w  w 

CALCAREOUS 
SANDSTONE. 

CALCAREOUS 
SHALE. 

MARL. 

BRECCIAS. 

The  word  breccia  is  of  Italian  origin  and  is  used  to  describe  ag- 
gregates of  angular  fragments  cemented  together  into  a  coherent 
mass.  The  breccias  cannot  all  be  properly  considered  to  be  either 
aqueous  or  eolian,  and  some  have  already  been  referred  to  under 
the  fragmental  igneous  rocks.  Oftentimes  they  resemble  con- 
glomerates, but,  unless  formed  of  fragments  of  some  soluble  rock, 
whose  edges  have  become  rounded  by  solution,  there  is  no  diffi- 
culty in  distinguishing  them.  Breccias,  as  regards  their  angular 
fragments  and  interstitial  filling,  may  be  of  the  same  materials  or 
of  different  ones.  We  may  distinguish  Friction  breccias  (Fault 
breccias),  Talus  breccias,  and,  for  the  sake  of  completeness,  may  also 
mention  here  Eruptive  breccias. 

Friction  breccias  are  caused  during  earth-movements  by.  the 
rubbing  of  the  walls  of  a  fault  on  each  other,  and  by  the  consequent 
crushing  of  the  rock.  The  crushed  material  of  finest  grade  fills  in 
the  interstices  between  the  coarser  angular  fragments,  and  all 
the  aggregate  is  soon  cemented  together  by  circulating  mineral 
waters.  Such  breccias  occur  in  all  rocks  and  are  a  frequent  source 
of  ores  which  are  introduced  into  the  interstices  by  infiltrating 
solutions.  Quartz  and  calcite  are  the  commonest  cements.. 

Talus  breccias  are  formed  by  the  angular  debris  that  falls  at  the 
foot  of  cliffs  and  that  becomes  cemented  together  by  circulating 
waters,  chiefly  those  charged  with  lime. 

Eruptive  breccias  may  be  produced  either  by  the  consolidation  of 
coarse  and  fine  fragmental  ejectments  like  tuffs,  or  by  an  erupting 
sheet  or  dike  that  gathers  in  from  the  wall  rock  sufficient  fragments 
as  inclusions  to  make  up  the  greater  part  of  its  substance.  These 
are  finally  cemented  together  by  the  igneous  rock  itself  and  afford 
curious  and  interesting  aggregates,  oftentimes  representing  all  the 
rocks  through  which  the  dike  has  forced  its  way  to  the  surface. 


6o  A   HANDBOOK  OF  ROCKS. 

A  crust  may  also  chill  on  a  lava  stream,  and  when  an  added  im- 
pulse starts  anew  the  flowing,  the  crust  may  be  shattered  into  an 
eruptive  breccia  of  a  still  different  type. 

We  often  speak  of  breccias  as  "brecciated  limestone,"  "brecciated 
gneiss,"  or  some  other  rock,  making  thus  the  character  of  the  original 
prominent.  When  the  fragments  and  the  cement  are  contrasted 
in  color,  very  beautiful  ornamental  stones  result,  which  may  be  sus- 
ceptible of  a  high  polish. 

A  moment's  consideration  of  the  above  methods  of  origin  will 
convince  the  reader  that  breccias,  except  as  formed  of  loose  vol- 
canic ejectments  are  of  very  limited  occurrence.  Although 
deeply  buried  rocks  that  share  in  profound  earth  movements  often 
suffer  crushing  and  brecciation  on  a  large  scale,  the  effects  are 
chiefly  detected  by  microscopical  study. 

GENERALITIES  REGARDING  SEDIMENTATION. 

In  the  production  of  the  rocks  next  taken  up,  moving  water 
plays  so  prominent  a  part  that  its  general  laws  are  described 
by  way  of  necessary  introduction.  All  streams  or  currents 
charged  with  suspended  materials  exercise  a  sorting  action  dur- 
ing the  deposition  of  their  loads.  With  materials  of  the  same 
density  the  sorting  will  grade  the  deposit  according  to  the  sizes  of 
the  particles.  With  materials  of  different  densities,  smaller  par- 
ticles of  heavier  substances  will  be  mixed  with  larger  particles  of 
lighter  ones.  Assuming  a  swift  current,  it  readily  appears  that, 
when  it  slows  up,  the  large  and  heavy  fragments  drop  first  of  all ; 
then  the  smaller  fragments  of  the  heavier  materials  and  the  larger 
ones  of  those  successively  lighter,  until  at  last  the  smallest  particles 
of  the  lightest  rock  alone  remain  in  suspension.  It  is  also  im- 
portant to  bear  in  mind  that,  the  density  being  the  same,  the  size 
of  the  transportable  particle  increases  with  the  sixth  power  of  the 
velocity.  Thus,  if  we  have  a  current  of  the  proper  velocity  it  will 
be  able  to  lift  a  grain  of  quartz  a  sixteenth  of  an  inch  in  diameter ; 
but  if  the  velocity  is  doubled,  the  transportable  particle  will  be  four 
inches  in  diameter.  An  appreciation  of  this  law  makes  the  size  of 
boulders  moved  by  many  streams,  in  times  of  flood,  less  surprising. 
On  the  other  hand,  when  the  suspended  material  becomes  exces- 
sively fine,  the  ratio  of  its  surface  to  its  volume  is  so  extremely 
high  that  adhesion,  or  chemical  action  akin  to  hydration,  or  some 
other  influence  not  well  understood,  operates  in  pure,  fresh  waters, 


ON  SEDIMENTATION.  61 

so  as  to  practically  render  sedimentation  impossible,  even  if  the 
medium  is  perfectly  quiet.  W.  M.  Brewer  has  shown  by  a  series 
of  experiments  with  all  sorts  of  clays,  lasting  over  many  years,  that 
if  we  introduce  into  such  an  emulsion  a  mineral  acid  or  a  solution 
of  salt,  or  of  some  alkali,  the  turbidity  clears  with  remarkable 
quickness.  When,  therefore,  sediment-laden  streams  flow  into  the 
ocean,  or  into  salt  lakes,  even  the  finest  part  of  their  load  speedily 
settles  out. 

While  we  may  state  thus  simply  the  laws  of  sedimentation,  we 
must  not  expect  in  Nature  such  well-sorted  and  differentiated 
results  as  would  at  first  thought  appear  to  be  the  rule.  Of  rivers 
and  shore  currents — the  two  great  transporting  agents — the  former 
are  subject  to  floods  and  freshets,  giving  enormously  increased 
efficiency  for  limited  periods,  and  again  to  droughts,  with  the  same 
at  a  minimum.  Hence  varying  sediments  overlap  and  are  involved 
together.  Eddies  and  quiet  portions  in  the  streams  themselves  con- 
tribute further  confusion,  and  an  intermingling  of  coarse  and  fine 
materials.  Shore  currents  have  parallel  increases  of  violence  in 
times  of  high  wind  and  storms,  and  sink  again  in  times  of  calm. 

Eolian  deposits  are  subject  to  even  greater  fluctuation,  and  their 
irregularities  are  more  pronounced  than  those  of  the  true  Aqueous. 
Both  these  classes  of  rocks  are  marked  by  a  more  or  less  perfect 
arrangement  of  their  materials  in  layers.  The  layers  give  rise  to 
regular  beds  in  deposits  from  quiet  and  uniform  currents,  and,  al- 
though in  those  from  swift  ones  they  are  very  irregular,  as  ex- 
plained above,  nevertheless  bedding,  or  stratification,  is  in  the  high- 
est degree  characteristic  of  the  Aqueous  and  Eolian  grand  division. 

When  in  the  presence  of  these  sedimentary  rocks  in  the  field, 
the  observer  should  always  appreciate  that  they  reproduce  past 
conditions,  and  that  they  indicate  the  former  presence  of  water, 
either  in  a  state  of  agitation  and  with  high  transporting  power  for 
the  coarse  varieties,  or  as  quiet  reaches  in  which  were  laid  down 
the  finer  deposits.  Rightly  appreciating  and  interpreting  them, 
we  may  see  that  the  ocean  has  advanced  across  the  land  in  times 
of  submergence,  leaving  behind  its  widening  trail  of  shore  gravels, 
now  conglomerates;  that  these  have  been  followed  up  and  buried 
first  by  fine  offshore  sediments,  and  later  by  the  remains  of  organ- 
isms now  appearing  as  limestones,  until  succeeding  elevation 
causes  the  waters  again  to  retreat  and  prepare  the  way  for  another 
"  cycle  of  deposition." 


62  A   HANDBOOK  OF  ROCKS. 


GRAVELS  AND  CONGLOMERATES. 

Loose  aggregates  of  rounded  and  water-worn  pebbles  and 
boulders  are  called  gravels,  and  when  they  become  cemented  to- 
gether into  coherent  rocks  they  form  conglomerates.  Sand  almost 
always  fills  the  interstices.  Silica,  calcite  and  limonite  are  the 
commonest  cement.  The  component  pebbles  are  of  all  sorts  of 
rock  depending  on  the  ledges  that  have  supplied  them,  hard  rocks 
of  course  predominating.  Rounded  fragments  of  vein  quartz  are 
especially  frequent.  Gravels  and  conglomerates,  if  of  limited  ex- 
tent, indicate  the  former  presence  of  swift  streams  ;  if  of  wide  area 
they  suggest  the  former  existence  of  sea  beaches  and  the  advance 
of  the  sea  over  the  land.  Component  pebbles  are  of  course  older 
than  the  conglomerate  itself,  and  if  igneous,  they  may  establish 
the  age  of  the  intrusion  as  older  than  the  conglomerate.  Fossil- 
iferous  boulders  prove  the  age  of  the  conglomerate  as  later  than 
their  parent  strata.  Under  favorable  circumstances  gravels  may  be 
cemented  to  conglomerates  in  a  comparatively  few  years.  Con- 
glomerates are  exclusively  aqueous.  Gravels  and  conglomerates 
graduate  by  imperceptible  stages  into  pebbly  sands  and  sandstones, 
and  these  into  typical  sands  and  sandstones.  Notably  unsorted 
aggregates  of  relatively  large  and  more  or  less  angular  boulders  in 
fine  sands  or  clay  indicate  glacial  action. 

Metamorphism.  Under  dynamic  stresses,  especially  in  the  na- 
ture of  pressure  and  shearing,  the  pebbles  of  a  conglomerate  may 
be  flattened  and  rolled  out  into  lenses,  and  such  are  often  observed. 
If  the  pebbles  are  feldspathic  as  is  the  case  in  those  from  granite 
ledges,  and  if  the  interstitial  filling  is  aluminous  and  not  purely 
quartzose  as  in  the  commonest  cases,  conglomerates,  when  re- 
crystallized,  may  pass  into  augen-gneisses  with  their  characteristic 
"  augen,"  or  "  eyes  "  of  feldspar  and  quartz  that  but  faintly  sug- 
gest their  original  character.  Excessive  metamorphism  may 
further  develop  types  closely  simulating  granite,  forming  thus  the 
so-called  "  recom^bsed  granite "  of  the  Lake  Superior  regions 

Occurrence.     Gravels  are  too  familiar  to  require  further  reference. 
Conglomerates  are  met  in  all  extended  sedimentary  series.     Our 
greatest  one  lies  at  the  base  of  the  productive  Coal  Measures  of 
Pennsylvania  and  adjacent  States.     It  is  properly  called  the  "  Grea<.< 
Conglomerate."     Remarkable  ones  with  squeezed  pebbles  are  met  ' 
in  the  Marquette  iron  region  of  Michigan.     In  Central  Massachu- 


SANDS  AND   SANDSTONES.  63 

setts  there  is  an  augen-gneiss,  that  has  been  derived  from  a  Cam- 
brian conglomerate.  It  is  quarried  at  Munson,  and  sold  as  granite, 
and  is  a  widely  known  building  stone.*  Around  Narragansett  Bay, 
R.  I.,  are  conglomerates,  in  part  at  least  of  Carboniferous  age,  in 
all  stages  in  the  progress  to  gneiss. 

SANDS  AND  SANDSTONES. 

FeO 

SiO2.     A12O3.        Fe2O3.     MnO.     CaO.     MgO.     K2O.     Na2O.      Loss.     Sp.  Gr. 

1.  99.78        0.22 

2.  98.84      0.17  0.34          tr.  tr.  tr.  .  .          .  .         0.23 

3.  99.62         .  .  0.13  0.7 

4.  99.47        O.I7  O.I  2  .    .          O.9O  0.50          O.O7          O.I5          O.I 2          2.648 

5.  95.85              264  .  .  0.81  0.08  .  .  .  .  0.45      (2.245) 

6.  94.73  0.36  2.64  .  .  0.38  0.36  .  .  .  .  0.83 

7.  91.67  6.92  tr.  .  .  0.28  0.34  .  .  .  .  1.17        2.240 

8.  82.52      7.07  3.55  1.42  1.83  tr.  tr.  tr.  3.61 

9.  69.94  13.15  2.48  0.70  3.09  tr.  3.30  5.43  i.oi       (2.36) 

I.  Sand  from  Cambrian  Quartzite,  Chesire,  Mass.,  S.  Dana  Hayes,  Mineral  Re- 
sources, i883~'84,  p.  962.  2.  Oriskany  Sandstone,  Juniata  Valley,  Penna.  A.  S. 
McCreath,  Idem.  3.  Siluro-Cambrian  saccharoidal  sandstone,  Crystal  City,~Mo.  Ana- 
lyzed by  Glass  Co.  0.22  not  determined,  Idem.  4.  Novaculite,  Rockport,  Ark.  R4 
N.  Brackett,  for  L.  S.  Griswold,  Geol.  Ark.,  1890,  III.  161.  5.  Salmon-red  Triassic 
Sandstone,  Glencoe,  Colo.  Quoted  by  G.  P.  Merrill,  "  Stones  for  Building  and  Deco- 
ration," p.  420.  The  Sp.  Gr.  is  of  one  from  Ralston,  near  by.  6.  Cambrian  red 
Sandstone,  Portage  Lake,  Mich.,  Idem.  7.  Light-gray  sub-carboniferous  sandstone, 
near  Cleveland,  Ohio,  Idem.  8.  Olive-green  carboniferous  sandstone,  Dorchester,  N. 
B.,  Idem.  9.  Red  triassic  sandstone  (brownstone,  arkose),  Portland,  Conn.,  Idem. 

Comments  on  the  Analyses.  The  first  three  illustrate  the  purity 
of  the  sand  in  exceptional  cases.  We  may  properly  infer  that  the 
sediments  were  derived  either  from  preexisting  sandstones,  that 
had  already  been  once  sorted  and  separated  from  their  aluminous 
ingredients,  or  from  excessively  weathered  and  kaolinized  quartzose 
rocks,  such  that  the  feldspar  had  entirely  passed  into  clay,  and  had 
been  eliminated  in  deposition.  No.  4  is  a  novaculite,  and  is  an  ex- 
cessively fine,  fragmental  deposit.  Nos.  5,  6  and  9  are  red  sand- 
stones, and  indicate  the  comparatively  small  percentage  of  iron 
oxides  that  may  cause  a  deep  coloration.  No.^  is  free  from  iron, 
but  has  some  aluminous  material,  evidently  a  very  pure  clay,  from 
the  lack  of  iron.  No. ^f  has  its  iron  as  protoxide,  for  the  rock  is  a 
green  variety.  Its  manganese  oxide  is  worthy  of  remark.  No. 

is  afeldspathic  sandstone,  or  arkose,  whose  analysis,  except  that 
the  A12O3  is  low  and  the  CaO  rather  high,  might  answer  for  a 
granite. 


64  A   HANDBOOK  OF  ROCKS. 

The  specific  gravity  of  sandstones  varies  widely.  Quartz  itself 
is  2.6-2.66,  and  specially  dense  sandstones  reach  2.5,  but,  being 
characteristically  porous  rocks,  the  usual  range  is  2.2-2.4.  They 
often  go  lower  and  may  even  reach  1.8. 

Miner alogical  Composition,  Varieties.  The  mechanical  sediments 
whose  predominant  particles  are  finer  than  pebbles,  and  yet,  in 
most  cases,  of  notable  size,  are  grouped  under  this  head.  They 
are  found  in  all  stages  of  coherence,  from  loose  sands  to  exces- 
sively hard  metamorphic  rocks  called  quartzites.  Quartz  is  much 
the  commonest  mineral  that  contributes  the  grains,  as  it  is  the  most 
resistant  of  the  common  rock-making  minerals.  In  river  sands  the 
grains  are  angular,  but  in  those  continually  swashed  together  on  a 
sea  beach,  they  become  more  or  less  rounded.  Garnets,  magnetite, 
zircon  and  other  hard  and  resistant  minerals  are  widely  distributed 
in  small  quantities.  Feldspathic  sands  also  occur,  and  when  they 
are  compacted  to  firm  rock  they  are  called  arkose.  As  in  the  con- 
glomerates, the  cementing  materials  of  sandstones  are  silica,  calcite 
and  limonite,  but  in  many  the  character  or  cause  of  the  bond  is 
rather  obscure.  Those  with  siliceous  cement  yield  the  most 
durable  stone  for  structural  purposes ;  those  with  ferruginous  afford 
the  greatest  range  of  colors,  such  as  olive-green,  yellow,  brown  and 
red.  Calcareous  cements  may  be  detected  by  their  feeble  efferves- 
cence. Sandstones  entirely  formed  of  calcareous  fragments  are 
known,  but  are  described  under  limestone. 

A  curious  and  exceptional  rock  is  novaculite,  that  is  extensively 
developed  in  Arkansas.  It  was  long  thought  to  be  allied  to  the 
cherts,  which  it  much  resembles,  but  microscopic  investigation  has 
led  Griswold  to  determine  it  to  be  a  finely  fragmental  deposit  of 
quartz  grains,  practically  a  siliceous  ooze.  In  fineness  it  is  parallel 
with  the  clays,  but  it  contains  little  else  than  silica. 

Aqueous  sandstones  generally  exhibit  well-marked  bedding 
planes,  although  cases  are  familiar  in  which  the  bedding  is  exces- 
sively coarse  and  the  layers  are  extremely  thick.  Swirling  eddies  in 
the  original  stream  or  currents  give  rise  to  cross-bedding  and  vari- 
ous irregularities.  In  fact,  all  the  phenomena  of  beaches  and 
stream-bottoms,  such  as  ripple -marks,  worm -borings,  shells,  etc., 
are  preserved  in  sandstones. 

Eolian  sands  are  usually  of  aqueous  deposition  in  their  original 
condition,  but  they  are  afterwards  taken  up  by  the  wind  and  driven 
along  as  dunes  and  dust  into  more  or  less  remote  districts.  When 


SANDS  AND  SANDSTONES.  65 

they  finally  reach  a  state  of  rest  and  consolidate  they  have  very* 
irregular  stratification,  cross-bedding,  swirling  curves,  pinching  and 
swelling  layers  and  other  characteristic  phenomena.  Finer  varieties 
afford  a  surface  deposit  that  is  generally  called  "  loess,"  and  that 
may  lack  all  stratification.  More  or  less  water-transported  mate- 
rial is  also  intermingled,  making  the  term  one  of  not  particularly 
sharp  definition.  This  mixed  character  has  made  the  loess  of 
many  localities  a  rather  puzzling  geological  problem.  It  is  always 
loosely  textured  and  is  important  in  its  relations  to  agriculture. 

Sands  and  sandstones  pass  by  insensible  gradations  into  the 
varieties  in  the  upper  line  of  the  series  shown  in  the  tabulation  on 
p.  59  by  the  increasing  admixture  of  clayey  or  argillaceous  ma- 
terials. The  base  is  kaolin,  A12O3,  2  SiO2,  2  H2O,  a  mineral  that 
forms  microscopic,  scaly  crystals  and  that  has  the  property  of  plas- 
ticity, and  this  property  it  lends  to  the  last  members  of  the  series, 
which  in  exceptional  cases  may  contain  little  else.  The  lower  series 
passes  gradually  into  the  fragmental  limestones,  by  the  increasing 
admixture  of  calcite. 

MetamorpJdsm.  The  purer  sandstones  in  metamorphism  yield 
quartzites  which  are  denser  and  harder  than  their  originals,  because 
by  deposition  of  cementing  quartz,  the  fragmental  grains  are 
very  firmly  bound  together.  The  later  'deposited  quartz  often 
conforms  to  the  optical  and  crystallographic  properties  of  the 
grain  around  which  it  crystallizes.  No  sharp  line  divides  sand- 
stones and  quartzites ;  they  shade  imperceptibly  into  one  another. 
Less  pure  sandstones,  if  crushed  and  sheared  in  the  metamorphic 
process,  yield  siliceous  or  quartz  schists  from  the  development 
of  micaceous  scales  between  the  grains.  Flexible  sandstone  or 
itacolumite,  appears  to  owe  to  them  its  property  of  bending. 

Occurrence.  Sandstones  are  so  common  in  all  extended  geologi- 
cal sections  as  to  deserve  slight  special  mention.  Next  to  lime- 
stones they  are  the  most  widely  used  of  building  stones  in  quantity, 
although  the  annual  value  of  granite  is  greater.  The  Potsdam 
sandstone  of  the  Cambrian  in  New  York  and  on  the  south  shore 
of  Lake  Superior  is  extensively  quarried,  and  other  prominent  ones 
are  the  Medina  of  New  York,  the  Berea  grit  of  the  Subcarbonifer- 
ous  of  Ohio ;  and  the  red  and  brown  Triassic  sandstones  both  of 
the  Atlantic  seaboard  and  the  Rocky  Mountains. 
5 


66 


A   HANDBOOK  OF  ROCKS. 


ARGILLACEOUS  SANDSTONE,  SHALE,  CLAY. 

FeO 


(a)SiO,  (b)SiO2 

MA 

Fe203 

CaO 

MgO 

K,O     Na.,O  (  a)  H,O  (  b)  H,O 

i.           63.31 

16.16 

3-79 

0.15 

4-44 

7.56      1.54 

265 

Shale. 

2.               69.92 

23.46 

020 

0.48 

0.40 

1-43 

3-84 

3-           67.29 

15.85 

6.16 

0-95 

0.19 

8.71 

.    4-          64.37 

19-73 

9.07 

0.82 

2.32 

3-78 

-  5.           62.86 

20.65 

9.21 

0.48 

o.34 

•  .  . 

6.26 

6.           58.45 

21.96 

8-43 

1.05 

1-57 

4.00 

651 

7-          43-13 

40.87 

3-44 

8.90 

5-32 

242 

0.20 

Brick.  Clay. 

8.            81.71 

9.81 

3.80 

0.48 

0.26 

.  . 

3-91 

9.           75.88 

11.22 

S.04 

0.48 

o-35 

. 

6.76 

10.           65.14 

I3-38 

7.65 

2.18 

2.36 

8.51 

.    . 

u.           62.00 

I8.IO 

9.  1  1 

.  . 

5.66 

12.               57.80 

22 

60 

1-85 

2.07 

12.68 

13-          53-77 

20.49 

9-23 

2.04 

4.22 

9.60 

'4-          45-73 

29.69 

6.86 

0.44 

I.OI 

3-42 

12.86 

Potters'  Clay. 

15.  27.68      36.58 

22.95 

1.28 

0.45 

o-37 

1.96 

6.74 

2-05 

16.  42.28      1802 

24.12 

1.46 

o-59 

0.68 

242 

7-77 

0.86 

Fire  Clay. 

17.           61.60 

28.38 

052 

046 

0.36 

5.08 

18.  38.10      12.70 

31-53 

0.92 

tr 

040 

11.30 

2.50 

19.           45.29 

40.07 

1.07 

0.26 

0.08 

0.48 

13.18 

Residual  Clay. 

20.           55.42 

22.17 

8.30 

0.15 

i-45 

2.49 

9.86 

21.          33-55 

30.18 

1.98 

3-89 

0.26 

i-57 

10.72 

22.              46.50 

39-57 

.  . 

.  . 

13-93 

NOTE.  Where  two  values  of  SiO2  are  given,  the  first  is  the  combined  silica,  i.  e., 
chiefly  in  kaolin,  and  the  second  the  free  silica,  which  is  practically  comminuted  quartz. 
Under  H2O,  where  two  values  are  given,  the  first  is  combined  water,  likewise  chiefly 
in  kaolin,  the  second  is  the  free  water,  which  has  simply  soaked  in. 
~^~i.  Argillaceous  sandstone,  Chickies  Station,  Penn.  R.  R.,  Geol.  Surv.  Penn. 
Rep.  M,  91.  2.  Haydensville,  Hocking  Co.,  O.  Quoted  by  H.  Ries,  XVI.  Ann. 
Rep.  Director  U.  S.  Geol.  Survey,  Part  IV.,  p.  572.  3.  Hornellsville,  Steuben  Co., 
N.  Y.,  Ibid.  572.  4.  Kansas  City,  Mo.,  Ibid.  570.  5.  Red  Shale,  Sharon,  Mercer 
Co.,  Pa.,  Ibid.  572.  6.  Leavenworth,  Kan.,  Ibid.  570.  7.  Clinton,  Vermilion  Co., 
Ind.,  Ibid.  570.  8.  Washington,  Daviess  Co.,  Ind.,  Idem.  566.  9.  Salem,  Washing- 
ton Co.,  Ind.  Idem.  566.  10.  Red  Clay,  Plattsburg,  Clinton  Co.,  N.  Y.,  Ibid.  568.  1 1.  Red 
Clay,  Lasalle,  111.,  Ibid.  564.  12.  Rondout,  N.  Y.,  Ibid.  568.  13.  Brown  Clay, 
Fisher's  Is.,  N.  Y.,  Ibid.  568.  14.  Hooversville,  Somerset  Co.,  Pa.,  Ibid.  568.  15. 
Akron,  O.,  Ibid.  562.  16.  East  Liverpool,  O.,  Ibid.  562,  alsoTiO.,,  1.20.  17.  Wood- 
bridge,  N.  J.,  Ibid  556.  18.  Cheltenham,  Mo.,  Ibid.  556.  19.  Woodland,  Pa.,  Ibid. 
556.  20.  Morrisville,  Calhoun  Co.,  Ala.,  Ibid.  574.  21.  Near  Batesville,  Ark.,  Ibid. 
574,  also  PA»  2-53-  22.  Pure  Kaolin— Al  A  2-SiO2,  aH2O. 

Comments  on  tJie  Analyses.  The  analyses  are  significant  when 
compared  with  those  of  the  sandstones  on  p.  63.  It  appears  at 
once  that  there  is  a  great  decrease  in  silica,  and  a  great  increase  in 
alumina,  and,  as  a  rule,  in  all  the  other  bases  and  water.  Among 


ARGILLACEOUS  SANDSTONES,  SHALES,  CLAY.     67 

themselves  there  is  wide  variation,  but  by  using  No.  22,  as  indi- 
cating pure  kaolin,  it  is  possible  to  infer  how  much  quartz  sand  is 
mingled  with  the  clay,  due  allowance  being  made  for  the  fragments 
of  unaltered  feldspar,  as  shown  by  the  alkalies,  for  silicates,  hydrous 
and  anhydrous,  involving  iron,  lime  and  magnesia,  and  for  carbo- 
nates of  lime,  magnesia  and  iron.  Shales  and  brick  clays  are  shown 
to  be  comparatively  impure  admixtures  of  kaolin  and  quartz ; 
potter's  clay  is  much  less  so,  and  fire  clay  is  little  else  than  these 
two.  No.  19  is  practically  pure  kaolin. 

Mineralogical  Composition.  ^  Varieties.  The  argillaceous  sandstones 
have  a  finer  grain  than  the  sandstones  proper,  and  tend  to  form 
thin  but  tough  beds.  They  find  their  best  examples  in  the  flag- 
stones of  our  eastern  cities.  Shales  lack  this  coherence  and  break 
readily  into  irregular  slabs  and  wedge-shaped  fragments  of  no  no- 
table size.  As  sands  give  rise  to  sandstones,  so  on  hardening  and 
drying,  muds  and  silt  yield  shales.  Shales  show  all  grades  from  gritty 
and  coarse  varieties  to  fine  and  even  ones  approximating  clays.  The 
finer  shales  when  ground  have  the  same  plasticity  as  clay,  and  are 
often  moulded  and  baked  into  brick,  especially  of  the  vitrified 
kinds  for  paving.  Shales  may  be  black  from  bituminous  matter 
in  them,  and  are  then  described  as  "bituminous."  They  grade 
into  cannel  coals,  but  great  areas  of  them  such  as  the  Genesee  Shale 
of  New  York  and  the  Huron  Shale  of  Ohio,  have  as  much  as  8  to 
2OC/0  hydrocarbons  and  yield  quite  copious  products  on  distillation. 

As  the  particles  of  quartz  become  finer  and  finer  and  not  too 
abundant,  the  plasticity  of  the  kaolin  presently  asserts  itself  so 
that  the  shales  pass  into  clays.  In  the  most  even  and  homo- 
geneous grades,  they  show  but  slight  grit  to  the  teeth,  but  in 
coarser  varieties  they  are  decidedly  gritty  even  to  the  fingers. 
They  are  often  separated  into  thin  beds  by  layers  of  sand  that 
mark  the  times  of  freshets  and  deposition  of  coarse  material  dur- 
ing their  formation.  Clays  of  earlier  geological  date  are  hard  and 
dense  rock  and  must  be  ground  before  use.  Such  are  the  fire-clays 
immediately  beneath  Carboniferous  coal-seams.  Clays  are  blue, 
red  and  brown  according  to  the  state  of  the  iron  oxide,  whether 
ferrous  or  ferric,  or  they  may  be  nearly  white  when  it  fails.  The 
less  pure  brick  clays  as  shown  by  the  analyses  contain  oxides  of 
iron,  calcium,  magnesium  and  of  the  alkalies  in  quantity,  but  fire- 
clays practically  lack  these. 

As  contrasted  with  the  transported  or  sedimentary  clays  just 
mentioned,  there  are  residual  clays  that  result  from  the  decay  of 


68  A   HANDBOOK  OF  ROCKS. 

impure  limestones  and  that  are  found  on  their  weathered  outcrops. 
They  are  very  impure  and  variable  in  composition,  but  they  are 
markedly  plastic. 

Metamorphism.  In  metamorphic  processes  shales  become  com- 
pacted and  oftentimes  silicified.  Their  lack  of  homogeneity  causes 
them  to  yield  irregularly  breaking  and  very  tough  rocks  called 
graywackes,  which  differ  only  in  greater  hardness  from  their 
unaltered  originals.  Excessively  silicified  shales  are  called  phtha- 
nites  and  are  important  in  the  Coast  Range  of  California.  Shales 
also  under  shearing  stresses  and  attendant  mineralogical  reorgani- 
zation pass  into  schists  of  various  kinds,  such  as  quartz-schist, 
mica-schist  and  possibly  hornblende-schist.  G.  F.  Becker  even 
mentions  rocks  derived  from  them  that  are  mineralogically  like 
diabases  and  diorites,  but  their  recognition  is  a  matter  for  micro- 
scopic study.  Clays  under  shearing  stresses  develop  new  cleavages 
without  regard  to  their  original  bedding  and  from  the  homo- 
geneous character  of  the  original  and  the  perfection  of  the  cleavage, 
slates  result,  which  are  of  great  practical  importance. 

Occurrence.  Shales  and  clays  are  such  common  members  of 
extended  geological  sections  as  to  deserve  no  special  mention. 
They  are  often  a  thousand  feet  or  more  in  thickness  and  cover 
great  areas. 

CALCAREOUS  SANDSTONES,  MARLS. 

H20or 
Na.,O        CO2       Loss 

3-26. 
0.29 


Calc 

. 

FeO 

Sandst.  SiO2 

AI20 

3 

Fe,O3 

CaO 

MgO 

K20 

I. 

79-19 

3.75 

7.76 

3-20 

f 

2. 

38.41 

5-77 

1.79 

20.08 

8.82 

0.  12 

Calc. 

Shales. 

3- 

39-7° 

26.83 

19.28 

2-43 

5.II 

4- 

28-35 

'2-37 

21.47 

8.24 

5-73 

Marls 

5- 

43-7° 

25.00 

8.85 

2-33 

.     . 

6. 

38-70 

10.20 

18.63 

9.07 

1.50 

3.65 

7- 

28.78 

11.63 

2.96 

24.50 

2.91 

2.12 

5.40       9.21 

6.14         10.00 
22.66          4.18 

i.  Calcareous  sandstone,  Flagstaff,  Ariz.  Quoted  by  G.  P.  Merrill,  Stones  for 
Building  and  Decoration,  420.  2.  Calcareous  sandstone,  Jordan,  Minn.,  Idem.  3. 
Genesee  Shale,  Mt.  Morris,  N,  Y.,  supplied  by  H.  Ries.  4.  Niagara  Shale,  Rochester, 
N.  Y.,  H.  Vulte,  analyst.  Supplied  by  H.  Ries.  5.  Cretaceous  Marl,  Hop  Brook,  N, 
J.  Geol.  of  N.  y.t  1868,419;  also  P2O5,  2.18.  6.  Cretaceous  Marl,  Red  Bank,  N. 
J.,  Idem,  418,-  also  P2O5,  1.14,  SO3  0.14.  7.  Subcarboniferous  marl,  Bowling  Green, 
Ky.  Ky.  Geol.  Surv.,  Chem.  Analyses  A,  Part  3,  90;  also,  P2O5,  0.25. 

Comments  on  the  Analyses.  The  analyses  illustrate  in  a  very 
suggestive  way  the  passage  of  these  mechanical  sediments  into  im- 
pure limestones.  The  gradual  intermingling  of  more  and  more  of 


CALCAREOUS  SANDSTONES,  MARLS.  69 

shells  and  other  remains  of  organisms  brings  it  about.  The  high 
P2O5  of  the  marls,  as  cited  under  the  references,  is  worthy  of  re- 
mark. It  is  to  be  appreciated  that  the  lime  and  magnesia  and 
some  of  the  iron  of  the  analyses  are  to  be  combined  with  CO2,  even 
though  the  CO2  is  not  mentioned. 

Mineralogical  Composition.  Varieties.  Calcareous  sandstones  are 
practically  sandstones  with  rich  calcareous  cement,  or  with  a  large 
amount  of  organic  fragments  intermingled  with  the  prevailing 
quartz  sand.  They  are  passage  forms  to  the  fragmental  limestones. 
Calcareous  shales  derive  their  lime  partly  from  the  fine  organic  sed- 
iment that  is  deposited  with  the  siliceous  and  aluminous  particles 
and  partly  from  contained  fossils.  Beds  of  these  rocks  are  partic- 
ularly favorable  layers  for  the  discovery  of  the  latter,  and  often 
break  the  monotonous  barrenness  of  a  geological  section  composed 
of  ordinary  shales.  Marls,  strictly  speaking,  are  calcareous  clays, 
and  originate  in  typical  cases  by  the  deposit  of  limy  slimes  along 
with  the  aluminous.  The  lime  destroys  the  plasticity  of  the  clay 
and  yields  a  crumbling  rock,  often  richly  provided  with  fossils  and 
of  value  as  a  fertilizer.  Grains  of  glauconite,  the  green  silicate  of 
potash  and  iron,  are  at  times  present,  and  characterize  the  so-called 
"  green  sands  "  that  are  valuable  as  fertilizers.  The  term  marl  is 
somewhat  loosely  used  in  its  applications,  and  moderately  coarse 
calcareous  sands,  and  even  beds  that  show  but  small  percentages 
of  lime  on  analysis  are  designated  by  it  in  the  States  along  the  At- 
lantic seaboard  from  New  York  south.  It  is  clear  that  marls  are 
intermediate  rocks  between  clays  and  impure  earthy  limestones. 

Metamorphism.  The  rocks  of  this  group  are  altered  in  metamor- 
phic  processes  to  schistose  forms,  not  so  essentially  different  from 
those  resulting  from  the  common  aluminous  shales  and  clays, 
except  that  the  richness  in  lime  facilitates  the  production  of  min- 
erals requiring  it.  The  marls,  when  high  in  lime,  behave  like  im- 
pure limestones,  and  are  prolific  sources  of  silicates.  Marls  are, 
however,  much  more  common  in  later  and  unmetamorphosed  for- 
mations than  in  older  ones,  although  it  may  be  that  in  the  latter 
they  have  yielded  some  schistose  derivatives  not  readily  traced 
back  to  them. 

Occurrence.  Calcareous  sandstones  and  shales  are  met  as  occa- 
sional beds  in  series  of  the  more  abundant,  distinctively  aluminous 
varieties.  Marls  are  chiefly  developed  in  the  Cretaceous  and  Tertiary 
strata  of  the  Atlantic  seaboard  and  around  the  Gulf  of  Mexico.  Fresh- 
water ones  are  not  lacking  in  the  Tertiary  lake  basins  of  the  West. 


CHAPTER  VIII. 

LIMESTONES;    ORGANIC    REMAINS,   NOT   LIMESTONES;   ROCKS 

PRECIPITATED  FROM  SOLUTION.  DETERMINATION  OF 

THE   AOUEOUS   AND   EOLIAN   ROCKS. 


II.     LIMESTONES. 

FeO 

SiO2         A12O3  Fe2O3 

CaO 

MgO     CO2 

H20 

Insol.    CaCO3 

MgCO, 

Living  Organisms. 

i.  (Coral) 

54-57 

2-54 

97.46 

2.  (Reef-rock) 

53-82 

I.OI 

96.11 

2.13 

3.  (Lagoon  Sed.) 

54.58 

.85 

97-47 

1.79 

4.  (Coral) 

44.96 

3-87 

80.29 

8.14 

5.  (Oyster  Shells) 

44-4 

i-3      35-4 

14-5 

(79.28) 

(2-73) 

Calcite. 

6.     Pure  Mineral. 

56. 

44. 

100. 

Dolomite. 

7.     Pure  Mineral. 

30-43 

21.72 

54-35 

45-65 

Marine  Limestones. 

8.       0.63                    0.55 

55-6 

0.23 

99.30 

0.49 

9.                                 1.06 

53-78 

0-34 

0.90 

1.13      96.04 

0.72 

10.                                          1.25 

53-89 

o.  10 

96.24 

O.2I 

ii.       1.84           0.64           1.82 

51.40 

2.23  41.19 

0.27 

91.80 

4.68 

12.     12.34                    7.00 

44.41 

0.44 

79-3° 

0.92 

I3-      3-77          °.°8          6.80 

33-79 

15.32  42.21 

60.35 

32.61 

FeO 

Si02         A1203  Fe20s 

CaO 

MgO     C02 

H20 

Insol.    CaCO3 

MgCOs 

H-                                 0.55 

29-54 

21.08 

1.82 

0.60      52.75 

44-28 

Waterlime. 

15.      18.34                     7.49 

37.60 

1.48 

3-94 

67.H 

2.90 

16.      15.37                   11.38 

25.70 

12.44 

1.20 

45-91 

26.14 

Siliceous. 

17.                                  1.20 

17.69 

10.59 

1.24 

43.72      31.60 

22.24 

Freshwater  Limestone. 

18.                                           0.37 

54.16 

0.15  43.68 

1.49      96.71 

0.31 

19.                                  1.83              0.22 

34-20 

o.i  i  26.79 

4.64 

31.28      61.07 

0.23 

Travertine. 

20.       0.08                     0.15 

53-83 

0.90  41.79 

i-43 

94-97 

0-43 

I.  Stag's  horn  coral,  {Millepora  alcicornis^,  S.  P.  Sharpless,  Amer.  Jour.  Sci. 
Feb.,  1871,  168.  2.  Bermuda  coral  reef  rock.  A.  G.  Hogbom,  Neues.  Jahrb.,  1894, 
I.  269.  3.  Bermuda  coarse  lagoon  sediment,  Idem.  4.  Average  of  14  analyses  of  the 
coral  Lithothamnium  from  localities  the  world  over,  Idem,  272.  5.  Oyster  shells,  GcoL 

70 


LIMESTONES.  71 

of  Xew  Jersey,  1868,  405.  6.  Calculated  from  CaCO3.  7.  Calculated  from  CaCO3, 
MgCO3.  8.  Crystalline  Siluro-Camb.  limestone,  Adams,  Mass.,  E.  E.  Olcott  for  Marble 
Co.  9.  Limestone,  Bedford  limestone,  Ind.  Quoted  by  T.  C.  Hopkins,  Mineral  In- 
dustry, 1894,  505.  10.  Solenhofen  lithographic  stone.  Quoted  by  G.  P.  Merrill,  Stones 
for  Building  and  Decoration,  415.  II.  Limestone,  Hudson,  N.  Y.,  Th.  Egleston. 
12.  Trenton  Limestone.  Point  Pleasant,  Ohio,  vide  No.  10.  13.  Surface  Rock,  Bonne 
Terre,  Mo.,  J.  T.  Monell,  unpublished.  14.  Limestone,  Chicago,  T.  C.  Hopkins, 
Mineral  Industry,  1895,  508.  15.  Hydraulic  limestone,  Coplay,  Penn.  Quoted  by 
W.  A.  Smith,  Mineral  Industry,  1893,  49.  16.  Hydraulic  limestone,  Rosendale,  N. 
Y.,  Idem.  17.  Siliceous  limestone,  Chicago,  111.,  vide  No.  14.  18.  Miocene  lime- 
stone, Chalk  Bluffs,  Wyo.,  R.  W.  Woodward,  4Oth  Parallel  Surv.  I.  542.  19.  Eocene 
imestone,  Henry's  Forks,  Wyo.,  B.  E.  Brewster,  Idem.  20.  Travertine,  below  Hotel 
Terrace,  Yellowstone  Park,  J.  E.  Whitfield,  for  W.  H.  Weed,  gfA  Ann.  Rep.  Dir. 
U.  S.  Geol.  Surv.,  646. 

Comments  on  the  Analyses.  The  first  three  and  the  fifth  analyses 
indicate  that  the  calcareous  parts  of  living  organisms  are  quite  pure 
calcium  carbonate.  The  fourth  analysis  is  of  that  species  of  coral 
which,  so  far  as  we  know,  is  highest  in  magnesia.  Small  amounts 
of  calcium  phosphate  are  often  present  as  well,  some  shells  being 
richer  than  others.  Nos.  6  and  7  are  introduced  so  as  to  give  a 
basis  for  estimating  the  purity  of  the  following  limestones:  Nos. 
8,  9  and  10  are  extremely  pure  varieties,  and  from  these,  as  a  start- 
ing point,  the  other  components  increase  in  one  analysis  and 
another.  No.  14  is  a  nearly  typical  dolomite.  Nos.  12  and  17  are 
highly  siliceous,  and  Nos.  15  and  16  are  both  strongly  argillaceous. 
The  last  two  are  closely  parallel  in  composition  with  marine  varie- 
ties. An  analysis  of  a  travertine  is  given  in  No.  20. 

It  at  once  appears  that  Nos.  13,  14,  16  and  17  are  far  higher  in 
magnesia  than  any  known  living  organism,  and  it  is  evident  that 
an  original  organic  deposit  must  have  undergone  an  enrichment  in 
magnesium  carbonate  to  bring  them  about.  Dana  suggested  many 
years  ago  that  coral  or  other  organic  sand,  while  agitated  in 
sea-water,  probably  exchanges  apart  of  its  calcium  for  magnesium, 
and  there  is  much  reason  to  think  that  it  does.  Otherwise,  the 
change  must  have  been  brought  about  by  magnesian  solutions  per- 
colating through  the  rock  and  altering  it  by  the  replacement  pro- 
cess called  dolomitization,  or  dolomization.  Much  of  the  silica,  no 
doubt,  results  from  radiolarians  and  sponge  spicules,  but  much 
also,  together  with  the  alumina,  from  fine  fragmental  sediments. 

Origin.  Much  the  greater  number  of  the  important  limestones 
are  of  marine  origin,  but  in  certain  geological  formations  fresh- 
water ones  are  well  developed.  The  calcareous  remains  of  organ- 


72  A   HANDBOOK  OF  ROCKS. 

isms  have  been  their  principal  source,  and  of  these  the  forami- 
nifera,  the  corals,  and  the  molluscs  the  chief  contributors.  Their 
shells  have  often  become  thoroughly  comminuted  to  a  calcareous 
slime  before  final  deposition,  so  that  the  resulting  rock  affords  no 
trace  of  organic  structure.  The  solubility  of  the  carbonate  of  lime 
aids  in  the  cementation  of  the  slime  to  rock  and  tends  to  efface  the 
organic  characters.  Limestones  pass  by  insensible  gradations 
through  more  and  more  impure  varieties  into  calcareous  shales  and 
marls,  but,  as  a  rule,  they  are  deposited  in  deeper  water  than  the 
true  shales  and  sandstones.  This  conception  must  not  be  applied 
too  strictly,  because,  beyond  question,  a  depth  of  a  few  feet  has 
often  sufficed,  and  too  much  emphasis  has  often  been  placed  upon 
the  depth  regarded  £s  necessary  for  limestones.  Coral  sands  ac- 
cumulate on  or  near  the  immediate  shore,  and  may  even  be 
heaped  up  by  the  wind. 

In  confined  estuaries  of  sea  water  subjected  to  evaporation, 
enough  carbonate  of  lime  is  precipitated  directly  from  solution,  to 
yield  important  strata,  and  such  are  often  met  in  a  series  of  beds 
associated  with  rock  salt  and  other  precipitated  rocks  as  later  set 
forth.  Calcareous  deposits  from  limy  springs  may  also  almost 
reach  the  dignity  of  rocks,  and  when  abundant  are  called  travertine 
or  calcareous  tufa.  If  particles  of  dust,  etc.,  are  suspended  in  limy 
springs  or  in  concentrated  estuarine  waters,  they  gather  concentric 
shells  of  the  carbonate  and  may  yield  oolitic  deposits  from  the  co- 
alescence of  the  concretions.  Some  algae  likewise  secrete  oolitic 
calcite  and  contribute  extensively  to  rocks. 

Mineral  Composition.  Varieties.  Calcite  is  the  chief  mineral  of 
limestones,  and  when  thin  sections  are  magnified  it  exhibits  its 
characteristic  cleavages.  Dolomite  and  siderite  accompany  it  fre- 
quently, and  their  molecules  also  replace  the  calcium  carbonate,  in 
a  greater  or  less  degree,  to  form  double  carbonates.  An  unbroken 
series  can  readily  be  traced  from  pure  calcium  carbonate,  through 
more  and  more  magnesian  forms,  to  true  dolomite.  Those  with 
over  5  </0  MgO  are  usually  described  as  magnesian  limestone,  and 
when  the  MgO  mounts  well  toward  the  21.72%  in  the  mineral 
dolomite,  we  use  the  latter  name.  In  the  same  way,  a  series  of 
ferruginous  varieties  may  be  established  toward  the  clay  ironstone 
and  black-band  ores,  and  a  siliceous  series  toward  the  flints  and 
cherts.  Cherty  limestones  are  a  very  common  variety,  and  are 
referred  to  again  in  connection  with  chert.  When  the  argilla- 


LIMESTONES.  73 

ceous  or  clayey  intermixtures  enter,  argillaceous  cr  hydraulic 
varieties  result  that  are  generally  drab  and  close-grained,  and  are 
useful  in  the  manufacture  of  cement.  Bituminous  matter  may  be 
present,  making  the  limestones  black,  and  this,  in  the  form  ol 
asphalt,  may  yield  asphaltic  varieties. 

Besides  these  varieties  established  on  the  basis  of  chemical  com- 
position, special  names  may  be  given  because  of  structure.  Thus 
earthy  limestones  tend  to  crumble  to  dirt ;  oolitic  limestones  re- 
semble the  roe  of  fish ;  pisolitic  varieties  consist  of  concretions 
of  size  comparable  with  peas;  and  other  terms  are  employed,  that 
are  self-explanatory.  Prominent  fossils  suggest  names,  such  as 
crinoidal,  from  fossil  crinoids;  coralline,  foraminiferal  and  many 
more  of  local  or  stratigraphic  significance.  Practical  applications 
play  a  part  in  nomenclature,  supplying  "  waterlime,"  "  cement- 
rock,"  ''  lithographic  limestone,"  etc. 

Metamorphism.  Limestones  feel  the  effects  of  metamorphism 
with  exceptional  readiness  and  under  deforming  stresses,  probably 
accompanied  by  elevation  of  temperature,  and  in  the  presence  of 
water,  or  along  the  contacts  with  intruded  dikes  and  sheets  of 
igneous  rocks,  they  lose  their  sedimentary  characteristics,  such  as 
bedding-planes  and  fossils,  and  change  into  crystalline  marbles. 
The  contained  bituminous  matter  becomes  graphite ;  the  alumina 
and  silica  unite  with  the  lime,  magnesia  and  iron  to  give  various 
silicates.  Other  oxides  together  with  the  bituminous  ingredients 
contribute  to  the  various  colorations.  Mechanical  effects  are 
manifested  in  flow  lines,  brecciation  and  other  familiar  features  of 
many  that  are  cut  and  polished  for  ornamental  stones.  Impure 
limestones  that  undergo  these  metamorphic  changes  are  the  most 
prolific  of  all  rocks  in  variety  and  beauty  of  minerals.  Arendal, 
Norway,  and  the  crystalline  limestone  belt  from  Sparta,  N.  J., 
north  through  Franklin  Furnace  are  good  illustrations.  The  crys- 
talline limestones  will  be  again  mentioned  under  the  metamorphic 
rocks. 

Occurrence.  Limestones  are  too  common  to  deserve  special 
mention  as  regards  occurrence.  They  are  common  rocks  in  all 
parts  of  the  country,  but  the  Trenton  limestone  of  the  Ordovician, 
the  Niagara  of  the  Silurian  and  the  Sub-carboniferous  limestones 
of  the  Mississippi  Valley  .are  specially  worthy  of  note. 


74  A    HANDBOOK  OF  ROCKS. 

III.  REMAINS  OF  ORGANISMS  NOT  LIMESTONES. 

Calcareous  remains  are  much  the  most  important  of  the  contri- 
butions made  by  organisms  to  rocks,  but  there  are  others,  respect- 
ively siliceous, ferruginous  and  carbonaceous,  that  deserve  mention. 

SILICEOUS  ORGANIC  ROCKS. 

The  principal  members  of  this  group  are  infusorial  or  diatoma- 
ceous  earths;  siliceous  sinters;  and  cherts,  hornstones  or  flints,  the 
three  last  names  being  practically  synonymous.  Infusorial  earths 
consist  of  the  abandoned  frustules  of  diatoms,  which  are  micro- 
scopic organisms  belonging  to  the  vegetable  kingdom.  Though 
not  a  common  rock,  they  yet  are  met  in  series  of  sedimentary 
strata,  both  freshwater  and  marine,  with  sufficient  frequency  to 
justify  their  mention.  Some  foreign  earthy  materials  are  unavoid- 
ably deposited  with  them.  The  siliceous  sinters  are  extracted  from 
hot  springs  by  algae  that,  as  shown  by  W.  H.  Weed,  are  capable  of 
living  and  secreting  silica  in  waters  up  to  i85°F.  They  are  far 
less  important  geologically  than  the  infusorial  earths.  Chert  is  a 
rock  consisting  of  chalccdonic  and  opaline  silica,  one  or  both.  It 
possesses  homogeneous  texture  and  is  usually  associated  with 
limestones,  either  as  entire  beds,  or  as  isolated,  included  masses. 
It  often  has  druses  of  quartz  crystals  in  cavities,  and  in  thin  sections 
under  the  microscope  it  sometimes  exhibits  sponge  spicules. 
Cherts  not  provided  with  these  organic  remains  may  be  regarded 
with  great  reason  as  chemical  precipitates,  and  as  American  varie- 
ties in  the  great  majority  of  cases  lack  them  the  cherts  receive 
more  extended  mention  under  the  chemical  precipitates. 

Infus.  Earths.  SiO2.  A12O3.  Fe2O3.      FeO.  CaO.  MgO.  Na,O.  K2O.  H2O. 

1.  91-43  2.89  0.66  0.36  0.25  0.63  0.32  3.8 

2.  86.90  409  1.26  0.14  0.51  0.77  0.41  5.99 

3.  75-86  9.88  2.92  0.29  0.69  0.08  0.02  8.37 
Silic.  Sinter. 

4.  8954  2.12  tr.  1.71  tr.  1. 12  0.30  5.13 
Chert.  ' . '  CaCO3.  MgCO3.  * — 

5-  34-0  0.80  63.4          1.5  03 

I.  Miocene,  Little  Truckee  River,  Nev.,  R,  W.  Woodward,  4Oth  Parallel  Survey, 
I.  opposite  p.  542.  2.  Fossil  Hill,  Nev.,  Idem.  3.  Richmond,  Va.,  M.  J.  Cabell,  Min- 
eral Resources,  1883-84,  p.  721.  4.  Deposit  from  Old  Faithful,  Yellowstone  Park, 
J.  E.  Whitfield,  for  W.  H.  Weed,  9th  Ann.  Rep.  Dir.  U.  S.  Geol.  Sur.,  670.  5.  Creta- 
ceous chert,  England,  Jukes-Brown  and  Hill,  Quar.  Jour.  Geol.  Soc.,  Aug,  1889. 


SILICEOUS  ORGANIC  ROCKS.  75 

Comments  on  the  Analyses.  The  infusorial  earths  are  fairly  high 
in  water,  and  this  is  the  main  cause  of  low  silica,  but,  as  stated 
above,  their  growth  and  accumulation  in  water  make  it  unavoid- 
able that  more  or  less  clay  and  other  sediments  should  mingle  with 
them.  In  these  and  the  other  members  of  the  series,  it  is  im- 
portant to  understand  that  much  of  the  silica  is  opaline,  or  amor- 
phous, hydrated  silica,  and  not  quartz  or  chalcedony.  Tests  of  the 
amounts  soluble  and  insoluble  in  caustic  alkali  are  usually  made  to 
determine  the  proportions  of  the  two,  for,  while  it  is  not  an  accurate 
separation — quartz  and  chalcedony  being  themselves  somewhat 
soluble — it  gives  an  approximate  idea.  No.  4  is  a  deposit  sepa- 
rated from  the  geysers  by  algae.  No.  5  is  largely  due  to  sponge 
spicules,  mixed  in  with  chalk,  and  therefore  is  high  in  calcic  car- 
bonate. 

Mineralogical  Composition.  Varieties.  The  mineralogy  of  the  in- 
fusorial earths  can  be  stated  less  definitely  than  the  chemical  com- 
position. The  individual  diatoms  are  very  minute,  but  the  analyses 
indicate  both  opaline  and  chalcedonic  silica  as  being  present.  In 
the  sinters  and  cherts,  when  the  latter  can  be  recognized  as  or- 
ganic, the  same  two  varieties  are  recognizable,  and  with  them  are 
varying  amounts  of  calcite.  The  infusorial  earths  are  fine,  pow- 
dery deposits,  resembling  white  or  gray  dried  clays,  but  they  lack 
plasticity  and  are  best  recognized  with  the  microscope.  Siliceous 
sinters,  often  called  geyserite,  are  cellular  crusts  and  fancifully 
shaped  masses  that  closely  resemble  calcareous  tufas,  but  that  are 
readily  distinguished  by  their  lack  of  effervescence.  Chert  is  dense, 
hard  and  homogeneous,  and  of  white,  gray  or  black  color.  It 
readily  strikes  fire  with  steel,  and  when  it  breaks  has  a  splintery  or 
conchoidal  fracture.  It  is  often  decomposed  to  powdery  silica  on 
the  outside,  and  in  extreme  cases  may  yield  rather  large  deposits 
of  this  powder,  that  are  called  "tripoli,"  and  are  used  for  various 
practical  purposes.  Mention  may  again  be  made  of  the  cherts  that 
seem  best  explained  by  chemical  precipitation. 

Metarnorphism.  The  cherts  alone  of  these  rocks  are  of  sufficient 
importance  to  attract  attention  in  this  connection,  and  their  meta- 
morphism  is  briefly  referred  to  on  page  81. 

Occurrence.  Infusorial  earths  are  abundant  near  Richmond,  Va., 
and  on  Chesapeake  Bay,  at  Dunkirk,  and  Pope's  Mills,  Md.  Beds 
deposited  in  evanescent  ponds  or  lakes  are  also  well  known  in 
States  further  north.  In  the  West,  the  Tertiary  strata  have  yielded 


76  A   HANDBOOK  OF  ROCKS. 

them  in  Nevada.  In  California  and  Oregon  great  areas  are  re- 
ported by  Diller.  Siliceous  sinters  produced  by  algae  are  quite  ex- 
tensive in  the  Yellowstone  Park,  and  similar  deposits,  perhaps 
caused  by  the  same  agent,  are  found  in  many  hot  spring  regions. 
Sinters  chemically  precipitated  also  occur.  The  most  important 
occurrences  of  chert  are  all  mentioned  together  on  page  81  . 

FERRUGINOUS  ORGANIC  ROCKS. 

It  may  be  questioned  if  these  deserve  the  dignity  of  rocks,  for 
they  may  with  great  propriety  be  confined  to  the  minerals  dis- 
tinctively so-called.  It  will  therefore  only  be  mentioned  that 
many  have  attributed  the  formation  of  beds  of  limonite  to  the 
separation  of  iron  hydroxide  by  low  forms  of  organisms. 
Even  granting  this,  it  is  still  true  that  such  limonites  are  insignifi- 
cant when  compared  with  those  that  result  by  purely  inorganic 
reactions  in  the  decay  of  rocks.  Important  strata  of  cherty  car- 
bonates of  iron  are  present  in  the  iron  mining  districts  around 
Lake  Superior  and  have  been,  no  doubt,  the  principal  source  of 
the  hematites.  Van  Hise  regards  them  as  probably  of  organic 
origin,  but  the  evidence  is  not  decisive  and  they  may  be  chemical 
precipitates.  Clay-ironstone  and  black-band  ores,  that  is,  argil- 
laceous and  bituminous  ferrous  carbonate,  sometimes  form  con- 
tinuous beds  instead  of  the  usual  isolated  lenses,  but  when  they  do, 
they  are  not  organic  in  origin,  although  decaying  organic  matter 
may  contribute  to  preserve  the  reducing  conditions  that  are  neces- 
sary to  the  formation  of  the  ferrous  salt. 

CARBONACEOUS  ORGANIC  ROCKS. 

When  plant  tissue  accumulates  in  damp  places  and  under  a  pro- 
tecting layer  of  water  that  prevents  too  rapid  oxidation,  new  ac- 
cessions may  more  than  compensate  for  loss  by  decay  so  that  ex- 
tensive deposits  may  result.  These  become  progressively  rich  in 
carbon  by  the  loss  of  their  other  elements  and  yield  beds  of  con- 
siderable geological,  but  much  greater  practical  importance.  The 
course  of  the  changes  and  the  several  stages  are  indicated  in  the 
following  table. 

C.          H.  O.  N.  Total. 

Woody  Tissue 50  6.  43  I.  100. 

Peat 59  6.  33.  2.  100. 

Lignite .  69  5.5          25.  0.8  100.3 

Bituminous  Coal 82  5.  13.  0.8  100.8 

Anthracite. 95.          2.5  2.5       trace.          100. 


CARBONACEOUS  ROCKS.     PRECIPITATES.          77 

The  changes  are  in  the  nature  of  loss  of  oxygen  and  hydrogen, 
and  also  of  carbon,  but  the  decrease  of  the  first  two  is  relatively  so 
much  greater,  that  the  carbon  actually  is  enriched.  The  table  is 
theoretical  in  that  no  account  is  taken  of  the  more  or  less  fortuitous 
mineral  matter  that  forms  the  ash  together  with  a  small  percentage 
of  incombustibles  in  the  vegetable  tissue  itself.  Peat  is  a 
more  or  less  incoherent  mass  of  twigs  and  stems,  decidedly  car- 
bonized and  darkened,  but  with  the  original  structures,  as  a  general 
rule,  still  well  preserved  and  recognizable.  By  gradual  stages  it 
passes  into  lignite,  which  is  still  further  compacted,  and  which  ex- 
hibits the  original  structures  more  faintly.  In  bituminous  coal,  they 
are  seldom  recognizable,  and  the  aggregate  is  compact  and  black.  In 
anthracite  the  coal  is  dense,  amorphous  and  lustrous.  The  oxida- 
tion necessary  to  the  latter  varieties  may  have  been  largely  per- 
formed before  actual  burial  in  other  rocks,  but  the  changes  are  con- 
tinuous and  progressive  in  all. 

Other  organic  derivatives,  such  as  asphalt,  petroleum,  etc.,  are 
not  considered  of  sufficient  abundance  to  rate  as  rocks. 

Metamorphism.  Anthracite  is  locally  produced  from  bituminous 
coal,  near  igneous  intrusions  and  by  regional  metamorphism,  as 
later  explained.  The  chemical  changes  are  the  same  as  those  pro- 
gressive ones  above  outlined,  but  are  doubtless  more  rapidly 
brought  about.  Anthracites  become  graphitic,  and,  as  a  theoret- 
ical extreme,  pass  into  graphite.  Natural  cokes  are  also  pro- 
duced along  intruded  dikes. 

Occurrence.  Peat  favors  cool  and  moist  latitudes  in  all  parts  of 
the  world,  and  is  chiefly  of  fresh  water  origin.  Lignites  and  coals 
are  best  developed  in  the  Carboniferous  and  Cretaceous  strata,  and 
where  the  former  occur  in  the  East  and  the  latter  in  the  West, 
they  often  contain  coal  seams. 

I\£.  PRECIPITATES  FROM  SOLUTION. 

The  narne  of  this  group  indicates  the  character  of  the  rocks 
that  comprise  it.  Bearing  in  mind  the  condition  established  at 
the  outset,  p.  3#;  that  a  rock  should  form  an  essential  part  of  the 
earth,  it  is  evident  that  water  is  the  only  natural  solvent  abundant 
enough  to  yield  such  rocks,  and  that  only  the  most  widespread 
compounds  that  are  notably  soluble  in  it,  or  in  its  common  solu- 
tions of  other  more  soluble  salts,  can  meet  this  requirement.  The 
rocks  may  be  conveniently  taken  up  under  the  following  heads. 


78  A   HANDBOOK  OF  ROCKS. 

I.   Precipitates    involving   the    alkaline   earths    and    alkalies.     2. 
Siliceous  precipitates.     3.  Ferruginous  precipitates. 

PRECIPITATES  INVOLVING  THE  ALKALINE   EARTHS  AND  ALKALIES. 

The  carbonate  of  lime  in  stalactites,  stalagmites  and  crusts  on 
the  walls  and  floors  of  caves  in  limestone  or  in  the  surface  deposits 
from  limy  springs  affords  a  rock  of  this  character,  that  is  a  form 
of  limestone,  from  pure  varieties  of  which  it  does  not  differ  in  com- 
position, although  its  banded  structure  and  rings  of  growth  which 
we  may  describe  by  Posepny's  useful  word  "  crustification,"  in  a 
measure  distinguish  it.  Naturally  such  deposits  are  often  beauti- 
fully crystalline,  free  from  admixture  except  of  associated  dissolved 
materials  and  as  a  rule  purer  than  sedimentary  limestones.  They 
yield  our  well-known  onyx  marbles.  Some  regularly  stratified  de- 
posits of  limestones  that  are  associated  with  the  precipitated  rocks 
next  discussed  have  doubtless  originated  together  with  them. 

Gypsum  and  Rock  Salt  are  the  chief  members  of  this  sub-group. 
They  occur  quite  invariably  in  association,  and  have  resulted  alike 
from  the  evaporation  of  sea-water  and  from  the  drying  up  of 
lakes,  originally  fresh.  Both  are  mixed  more  or  less  with  dust  and 
other  mechanical  sediments  washed  or  blown  into  the  evaporating 
reservoir,  or  are  interbedded  with  other  salts  that  were  present  in 
a  minor  capacity  in  the  mother  liquor,  but  instances  of  thick  beds, 
especially  of  rock  salt  of  surprising  purity,  are  well  known.  When 
these  attain  several  hundred  or  even  a  thousand  feet,  it  is  evident 
that  more  than  twenty-five  times  this  depth  of  salt  water,  on  the 
basis  of  the  known  composition  of  the  sea,  would  have  to  be  evap- 
orated, and  this  is  a  practical  absurdity  even  for  any  conceivable 
confined  body,  with  occasional  renewals  from  breaches  of  the  bar- 
rier. It  would  be  necessary  to  assume  wide  stretches  of  shallows 
that  were  practically  evaporated  to  dryness,  while  at  the  same  time 
subsidence  of  the  coast  was  progressing  at  just  about  the  necessary 
rate  to  keep  pace  with  the  growth  of  the  salt.  The  recent  ex- 
planation, however,  advanced  as  the  "  Bar  theory,"  by  Ochsenius*, 
clears  it  up.  We  need  only  to  assume  a  relatively  deep  and  nearly 
land-locked  estuary,  with  a  shallow  bar  between  it  and  the  sea. 
Evaporation  continually  concentrates  the  confined  salt  water  and 


*  Zeitschrift  f.  Praktische  Geologic.   May  and  June,  1 893.   An  excellent  abstract  by 
L.  L.  Hubbard  appears  in  the  Geol.  of  Michigan.  V.  Part  II.,  p.  ix. 


PRECIPITATES  FROM  SOLUTION.  79 

especially  the  portion  on  the  shallow  bar.  This  becoming  rich 
in  mineral  matter  and  of  high  specific  gravity,  flows  inward  and 
down  the  slope  of  the  bar  to  the  bottom  of  the  estuary.  In  the 
course  of  time,  and  allowing  for  the  influence  of  pressure  in  the 
depths  and  of  temperature,  conditions  favorable  to  precipitation, 
first,  of  the  insoluble  gypsum,  later  of  the  more  soluble  common 
salt  will  be  reached,  and  in  varying  and  alternating  layers  they  will 
be  built  up  indefinitely,  or  until  some  upheaval  or  subsidence  alters 
the  relations  of  the  estuary  to  the  sea.  More  or  less  anhydrite  is 
also  deposited,  and  is  later  found  in  extended  cross-sections  of 
salt-bearing  strata.  The  most  soluble  ingredients,  such  as  KC1, 
MgCl2,  MgSO4,  etc.,  become  continually  richer  in  the  mother 
liquor,  and  unless  this  is  finally  evaporated  too,  they  escape  and 
are  not  found  in  the  series.  So  far  as  we  know,  the  Stassfurt  dis- 
trict, in  Germany,  is  almost  the  only  place  where  this  escape  has 
been  prevented  on  a  large  scale,  although  rock  salt  is  of  world- 
wide distribution. 

Gypsum  forms  at  times  gray  or  black  earthy  beds,  that  look  very 
much  like  limestone,  but  of  course  do  not  effervesce.  Again,  it  is 
in  white,  cream-colored  or  deeper-tinted  layers,  yielding  alabaster. 
Minor  portions  afford  selenite,  the  clear,  transparent  variety,  and 
thin  coats  of  native  sulphur  are  seldom  lacking.  Rock  salt  forms 
crystalline  beds,  often  stained  red  or  brown,  by  iron-oxide.  Both 
gypsum  and  salt  may  impregnate  associated  sediments  more  or  less, 
yielding  gypseous  or  saline  shales  and  marls.  In  many  localities 
gypsum  deposits  have  undergone  a  complex  series  of  chemical 
changes  in  the  general  nature  of  deoxidization  from  carbonaceous 
matter  present,  so  as  to  yield  native  sulphur  in  large  amounts. 

Metamorphism.  None  of  the  above  rocks  are  worthy  of  mention 
as  regards  metamorphism. 

Occurrence.  In  America,  gypsum  is  found  especially  in  the  Upper 
Silurian  of  New  York;  the  Lower  Carboniferous  of  Michigan  and 
Nova  Scotia  ;  the  Triassic  in  the  Eastern  prairie  states,  Kansas  and 
Texas  ;  in  undetermined  Mesozoic  in  Iowa,  and  in  the  Jura-Trias 
or  in  undetermined  strata  in  Colorado,  Utah  and  the  West.  Rock 
salt  occurs  in  the  Upper  Silurian  of  southern  New  York;  in  the 
Triassic  of  Kansas ;  in  the  Quaternary  (?)  of  Petite  Anse,  La., 
and  at  many  places  of  recent  geological  age  in  the  West. 


8o 


A   HANDBOOK  OF  ROCKS. 


SILICEOUS  PRECIPITATES. 


Geyserite.  (a)SiO2.(b)SiO2,  A12O3.  Fe2O3.   CaO.    MgO.    K2O.  Na2O. 
I.                      8195                 6.49        tr.          0.56      0.15      065     2.56 

Cherts. 

2. 

99.46 

0.29 

0.4          tr. 

3- 

3-35     95-78 

o.i  6 

tr.          o.o  i 

4, 

4-52    9365 

0.83 

0.05      o.o  i 

5- 

98.10 

024    0.27 

o.i  8 

0.23 

6. 

94.91 

2.85 

0.42       tr. 

Sil.  Oolite. 

7- 

95-83 

2.03 

1.93        tr. 

CaCO3.  MgCO3. 

8. 

56-50 

1.50 

16.84    2.60 

9- 

3-70 

1.42 

88.71     8.09 

Cherty  iron 

carbonates. 

CaO.  MgO.    FeO. 

MnO. 

10. 

58-23 

0.06     5.01 

0.38    9-59    18.41 

0.25 

n. 

4646 

0.24    0.64 

1.87     3.10    26.28 

O.2I 

12. 

28.86 

1.29     i.oi 

0.74     3.64    37.37 

0.97 

Loss. 

7-5° 

o-34 

O.2O 

0.78 

1.16 


12.54 


2.08 

1.22 

0.68 


Sp.Gr. 


2.63 

2.688 
2.654 
C02. 
5.22 
19.96 
25.21 


NOTE.  (a)SiO2  means  silica  soluble  in  caustic  alkali ;  (b)SiO2,  silica  insoluble  in 
the  same. 

I.  Geyserite,  Splendid  Geyser,  Yellowstone  Park,  J.  E.  Whitfielcl  for  W.  H.  Weed, 
qth  Ann.  Rep.  Dir.  U.  S.  Geol.  Surv.,  670.  2.  Gray  unaltered  chert,  Joplin,  Mo.  An- 
alysis made  by  U.  S.  Geol  Surv.  Quoted  in  Ann.  Rep.  Geol.  Surv.  Ark,,  1896,  III. 
161.  3.  White  altered  chert,  Galena,  Kan.,  Idem.  4.  Unaltered  chert,  Bellville,  Mo., 
Idem.  5.  Decomposed  chert,  or  Tripoli,  Seneca,  Mo.,  W.  H.  Seamon.  Quoted  by  E.  O. 
Hovey,  Amer.  Jour.  Sci.,  Nov.,  1894,  406.  6.  Chert,  Roaring  Springs,  Newton  Co., 
Mo.,  J.  D.  Robertson,  for  E.  O.  Hovey,  Idem.  7.  Siliceous  oolite,  Center  Co.,  Penn., 
Barbour  and  Torrey,  Amer.  Jour.  Set.,  Sept.,  1890,  249.  8.  Silica-lime  oolite,  Idem. 

9.  Lime-silic  i  oolite  on  same  specimen  as  No.  8,  Idem.     10,  n.  Cherty  iron  carbo- 
nates, N.  E.  Minn.,  T.  M.  Chatard,  for  C.  R.  Van  Hise,  Monograph  XIX,  U.  S.  Geol. 
Survey,  192.     12.  Cherty  iron  carbonate,  Sunday  Lake,  Gogebic  Range,  Mich.,  W. 
F.  Hillebrand,  Idem. 

Comments  on  the  Analyses.  The  first  seven  are  high  in  silica, 
some  approximating  chemical  purity.  No.  I  has  admixtures  of 
mud  thrown  out  by  the  geyser  from  its  walls.  The  five  cherts 
2-6  inclusive  have  but  slight  amounts  of  alumina,  iron  and  lime, 
and  low  percentages  of  water.  Nos.  3  and  4,  by  the  determina- 
tions of  soluble  silica  give  us  some  idea  of  the  amount  of  the 
opaline  form  that  is  present-.  The  three  analyses  7,  8  and  9  are 
a  most  instructive  series,  passing  as  they  do  from  nearly  pure  silica 
into  a  moderately  siliceous,  magnesian  limestone,  from  which  the 
first  two  are  thought  to  have  been  derived  by  replacement.  Nos. 

10,  II  and  12  are  the  curious  cherty  carbonates  of  iron  from  which 
the  Lake  Superior  iron  ores  have  been  formed  by  subaerial  decay. 
Their  richness  in  magnesia  as  compared  with  lime  is  noteworthy. 


SI  LI  CEO  US  PRECIPITA  TES.  S  i 

Mineralogical  Composition.  Varieties.  Cherts  are  so  exceedingly 
fine  grained  that  they  give  no  indication  of  their  constituent 
minerals  to  the  unaided  eye.  The  microscope  shows,  however, 
that  they  are  chiefly  chalcedony  in  excessively  minute  crystals, 
with  which  are  associated  varying  amounts  of  opaline  silica,  quartz 
crystals,  calcite  or  dolomite  rhombs  and  dusty  particles  of  iron 
oxide.  In  foreign  cherts  as  stated  above  on  p.  74,  sponge  spic- 
ules  have  been  met,  but  not  in  the  important  American  varieties. 
Cherts  often  have  an  outer  powdery  crust  due  to  decomposition, 
and  while  as  shown  by  analysis  5,  this  may  not  mean  any  notable 
chemical  change,  it  may  penetrate  whole  beds  and  leave  only  a 
white  incoherent  mass  called  "tripoli,"  that  is  used  for  a  polishing 
powder  and  for  various  other  purposes.  Cherts  have  spherulites 
occasionally  and  are  still  more  often  oolitic.  The  cherty  or  silic- 
eous rocks  of  the  formations  containing  the  Lake  Superior  iron 
ores  are  mixtures  of  chalcedonic  silica  and  carbonate  of  iron  in 
varying  proportions,  and  in  their  alteration  they  afford  more  or  less 
sharply  differentiated  jaspers  and  hematites.  Three  analyses  of 
varying  composition  are  given  above,  Nos.  10,  n  and  12. 

As  stated  earlier,  cherts  are  intermingled  in  all  proportions  with 
limestones.  They  are  very  puzzling  problems  as  regards  origin. 
Where  devoid  of  organisms,  the  majority  of  observers  regard  them 
as  in  some  way  precipitated  chemically  from  sea  water,  possibly  by 
way  of  replacement  of  limestone.  Their  structure  and  relations 
give  us  few  definite  clues  on  which  to  base  a  firm  conclusion.  As 
earlier  stated,  others  regard  them  as  derived  from  siliceous  remains 
of  organisms,  such  as  sponges,  radiolarians  and  the  like,  which  may 
have  been  redissolved  and  worked  over  into  chalcedony,  making 
them  practically  precipitates.  Cherts  are  also  called  hornstone  and 
flint. 

Metamorphism.  Purely  siliceous  cherts  are  unpromising  sub- 
jects for  metamorphism,  except  as  they  yield  silica  in  cherty  lime- 
stones for  the  production  of  silicates.  The  ferruginous  cherts  of 
Lake  Superior  pass  into  actinolitic  and  magnetitic  slates,  a  most 
interesting  change,  especially  in  the  former  case.  The  lime,  mag- 
nesia and  iron  are  combined  with  silica  under  the  metamorphosing 
influences,  so  as  to  yield  the  actinolite. 

Occurrence.  The  abundance  of  cherts  or  related  rocks  in  the  re- 
gion of  Lake  Superior,  either  associated  with  limestone  or  in  the 
cherty  carbonates  described  above  is  remarkable.  In  their  eco- 
6 


82  A   HANDBOOK  OF  ROCKS. 

nomic  products,  they  are  the  most  important  strata  present.  The 
Siluro-Cambrian  limestones  are  often  cherty  both  east  and  west, 
and  in  the  New  York  and  Ohio  Devonian,  the  so-called  "  Cornifer- 
ous  "  limestone  was  named  from  its  richness  in  "  hornstone."  In 
the  Mississippi  Valley  the  lower  Carboniferous  strata  are  particu- 
larly prolific  in  cherts. 

FERRUGINOUS  PRECIPITATES. 

Some  iron  ores  doubtless  originate  in  this  way,  and  the  processes 
by  which  the  soluble  proto-salts  are  oxidized  and  precipitated  as 
the  insoluble  ferric  hydroxide  are  well  understood.  But  they  may 
'be  considered  rather  as  minerals  than  as  rocks.  The  cherty  iron 
carbonates  of  the  preceding  section  have  already  been  cited,  and 
the  clay  ironstones  and  black-band  iron  ores  are  omitted  from 
further  mention  for  the  same  reasons  as  are  the  limonites. 

THE  DETERMINATION  OF  THE  AQUEOUS  AND  EOLIAN  ROCKS. 

The  members  of  this  series  are  much  easier  to  recognize  than 
are  the  igneous.  Breccias,  conglomerates  and  sandstones  are  at 
once  apparent  from  their  fragmental  character.  Breccias  differ 
from  conglomerates  in  the  angular  shape  of  their  component  frag- 
ments. As  the  sandstones  become  finer,  the  argillaceous  varieties 
may  be  distinguished  by  the  peculiar  odor  emitted  by  all  clays  and 
clayey  rocks  when  breathed  upon.  The  calcareous  sandstones 
and  marls  betray  themselves  by  effervescence  with  acid.  All  lime- 
stones, unless  too  rich  in  magnesia,  effervesce  in  cold  acid,  and  the 
more  readily  if  first  scraped  up  into  a  little  heap  of  powder  with  a 
knife.  Dolomites  effervesce  much  less  readily,  and  warm  acid  may 
be  necessary.  Infusorial  earth  may  need  the  microscope  for  its  cer- 
tain identification,  and  then  the  abundance  of  the  little  organisms 
is  very  apparent.  The  cherts  are  so  characteristic  in  appearance 
as  to  admit  of  little  uncertainty,  except  as  compared  with  the  silici- 
fied  tuffs  and  excessively  fine  felsites,  called  petrosilex,  in  which  case 
geological  surroundings  or  the  microscope  are  the  only  resources. 
The  ferruginous  rocks,  if  such  be  allowed,  are  self-evident,  as  are  the 
carbonaceous.  Gypsum  is  easily  recognized  when  in  the  crystal- 
line form,  but  when  black  and  earthy,  the  observer  may  be  forced 
to  determine  its  lack  of  effervescence,  and  to  make  a  sulphur  test 
with  the  blowpipe.  Nevertheless  with  these  rocks  as  with  the 
igneous,  although  to  a  less  degree,  it  is  very  advisable  to  gain  ex- 


REMARKS  ON  DETERMINATION.  83 

perience  with  correctly  labeled  study  collections  or  with  the  syste- 
matic exhibits  of  a  museum,  so  that  the  observer  may  have  a  fund 
of  experience  back  of  him  from  which  to  draw,  and  on  which  to 
depend  when  a  rock  comes  up  for  determination. 

For  field  work  and  travel,  it  is  well  to  appreciate  that  a  few  dry 
crystals  of  citric  acid,  that  can  be  dissolved  in  a  little  water  as 
needed,  serve  very  well  for  tests  of  effervescence.  They  are  more 
safely  carried  than  are  liquid  mineral  acids. 


CHAPTER  IX. 

THE    METAMORPHIC   ROCKS.     INTRODUCTION.    THE    ROCKS    PRO- 
DUCED BY  CONTACT  METAMORPHISM. 

The  word  metamorphism  was  first  introduced  into  geological 
literature  by  Lyell  in  1832,  and  was  used  to  describe  the  pro- 
cesses by  which  rocks  undergo  alteration.  It  was  particularly  ap- 
plied by  him  to  those  stratified  rocks  that,  from  deep  burial  in  the 
earth,  and  from  the  consequent  heat  and  pressure  to  which  they 
have  been  subjected,  have  assumed  structures  and  textures  resem- 
bling those  of  the  unstratified  primary  or  plutonic.  In  this  sense 
it  has  been  generally  employed  since,  and  it  implies  an  increase  in 
crystallization,  hardness  and  those  attributes  which  are  especially 
associated  with  the  crystalline  schists,  as  contrasted  with  the  un- 
altered sediments. 

The  literal  meaning  of  the  phrase  "the  processes  by  which  rocks 
undergo  alteration "  may  nevertheless,  be  somewhat  more  com- 
prehensive than  this,  and  may  be  made  to  include  the  changes 
produced  by  atmospheric  agents,  which  we  ordinarily  describe  by 
the  term  weathering,  and  in  the  following  pages  the  products  of 
this  latter  form  of  alteration  will  be  briefly  considered  as  a  third 
and  concluding  group. 

The  metamorphic  "ytocks  will  therefore  be  taken  up  under  the 
following  three  classes. 

I.  Rocks  reduced  by  Contact  Metamorphism. 
II.  Rocks  produced  by  Regional  Metamorphism. 
III.  Rocks  produced  by  Atmospheric  Weathering. 


THE  METAMORPHIC  ROCKS.     INTRODUCTION.     85 

By  contact  metamorphism  is  meant  the  series  of  changes  that 
are  effected  by  an  igneous  intrusion,  such  as  a  dike  or  a  laccolite 
upon  the  rocks  through  which  it  is  intruded.  These  changes  are 
often  profound,  and  are  brought  about  by  the  heat  of  the  intrusion 
as  well  as  by  vapors  and  hot  solutions  which  it  may  likewise  give 
forth.  The  wall-rock  may  be  itself  igneous  or  sedimentary,  or  even 
metamorphic.  This  form  of  metamorphism  is  sometimes  called 
"local"  as  contrasted  with  "regional." 

By  regional  metamorphism  we  describe  the  series  of  changes 
that  are  produced  in  the  rocks  of  wide  areas  or  "  regions"  by  deep 
burial,  mountain-making  upheavals,  and  by  heat  and  pressure. 
Although  Lyell  had  stratified  rocks  before  him  as  the  chief  ma- 
terials on  which  these  agents  acted,  yet  it  is  well  recognized  to-day 
that  igneous  rocks  are  no  less  profoundly  affected,  and  indeed  that 
the  results  of  their  alteration  maybe  almost  or  quite  indistinguish- 
able, from  those  derived  from  sediments.  But  there  is  great  un 
certainty  as  to  the  original  condition  of  many  regionally  meta 
morphosed  rocks,  and  although  the  endeavor  has  been  made  in 
previous  pages  to  throw  as  much  light  upon  them  as  possible,  by 
systematically  referring  to  the  alteration  and  metamorphism  of 
simple  types,  nevertheless,  many  are  obscure,  and  in  their  history 
are  involved  some  of  the  profoundest  problems  of  geology. 

By  atmospheric  weathering  is  meant  the  series  of  changes 
wrought  in  rocks  at  or  near  the  surface  of  the  earth,  by  the  ordi- 
nary atmospheric  agents,  water,  oxygen,  carbonic  acid  and  the  like. 
The  changes  are  chiefly  in  the  nature  of  disintegration,  loss  of 
soluble  ingredients  and  decomposition,  and  in  general  they  pro- 
duce a  marked  shrinkage  of  bulk. 

It  is  important  to  appreciate  that  under  whatever  form  the  me- 
tamorphic  rocks  are  met,  they  are  of  necessity  alteration  products 
of  the  two  grand  divisions  over  which  we  have  already  passed. 

GENERALITIES  REGARDING  CONTACT  METAMORPHISM. 
Widening  observation  has  shown  that  contact  metamorphism  is 
produced  by  all  varieties  of  igneous  rocks,  and  that  it  may  be 
broadly  stated  to  be  independent  of  the  kind  of  rock  forming  the 
intrusion.  Granites,  syenites,  nepheline-syenites,  diorites,  gabbros 
and  even  peridotites  have  in  one  place  and  another  proved  to  be 
efficient  agents.  Yet  the  following  statements  may  be  said  to  hold 
good: 


86  A  HANDBOOK  OF  ROCKS. 

1.  Plutonic  rocks  are  more  favorable  to  it  than  volcanic.     This 
follows  because  plutonic  rocks  cool  slowly,  at  considerable  depths 
and   stand  therefore  at  high  temperatures  for  long  periods  next 
their  walls. 

2.  Magmas  rich  in  mineralizers  are  much  more  favorable  than 
are  those  poor  in  them.     This  naturally  follows  from  the  powerful 
influence  exerted  by  escaping  vapors.     It  is  tantamount  to  saying 
that  acidic  rocks  are  in  general  more  efficient  than  basic  ones, 
because  experiment  shows,  and  field  observation  indicates,  that 
abundant  absorbed  vapors  accompany  and  facilitate  the  fusion  of 
the  rocks  high  in  silica,  whereas  basic  rocks  are  much  more  largely 
the  results  of  dry  fusion.     Granites,  for  instance,  are  the  com- 
monest and  most  effective  agents  of  contact  metamorphism. 

3.  As  regards  the  walls,  sedimentary  rocks  possess  varying  sus- 
ceptibilities.    Highly  siliceous  sandstones  and  conglomerates,  for 
example,  are  stubborn  subjects,  and  manifest   but  slight  altera- 
tion ;  but  highly  aluminous  or  calcareous  beds  are  favorable  to  re- 
crystallization  because  they  contain  the  alumina,  iron,  lime,  mag- 
nesia and  the  alkalies  that  will  combine  with  silica,  under  meta- 
morphosing influences,  to   yield   copious    contact  minerals.     Of 
all  rocks,  impure  limestones  yield  the  most  varied  and  interesting 
results. 

4.  With  a  favorable  intrusion,  the  apparent  distance  to  which 
the  metamorphosing  influence  penetrates,  depends  on  the  angk  of 
emergence  of  the  intrusion.     If  it  comes  up  at  a  low  angle  it  may 
lie  but  a  short  distance  below  the  surface  for  a  considerable  stretch 
on  one  side  of  the  outcrop,  so  that  the  metamorphosed  area  may  ap- . 
parently  extend  to  a  great  distance,  although  at  no  point  fer  from 
the  source  of  heat.     Around  a  vertical  dike  the  distance  would 
naturally  be  less.    Again,  the  alterations  progress  much  less  readily 
across  the  bedding  of  stratified  rocks  than  along  it.     Hence,  an  in- 
trusion that  cuts  across  the  bedding  produces  more  widespread' 
effects  than  does  one  parallel  with  it. 

5.  It  is  believed  by  many,  especially  among  English  and'  Ger- 
man observers,  that  there  is  very  slight  migration  of  material  dur- 
ing metamorphism,  and  therefore  that  the  contact  minerals  have 
resulted  from  the  silica  and  the  bases  that  were  practically  in  the 
same  places  before  the  intrusion  as  after  it.     It  follows  that  there 
has  been  no  chemical  introduction  or  substitution,  but   only  re- 
arrangement of  molecules  during  the  process.     An  analysis,  there- 


7 HE  METAMORPHIC  ROCKS.     INTRODUCTION.     87 

fore,  of  a  reasonably  large-sized  sample  would  indicate  the  compo- 
sition of  the  original  rock,  except  so  far  as  water,  carbonic  acid, 
and  other  volatile  ingredients  have  been  driven  off.  From  obser- 
vations upon  an  intrusion  of  granite  in  Westmoreland,  England, 
that  cuts  a  decomposed,  basic,  amygdaloidal  lava,  Alfred  Harker 
concluded  that  the  migration  had  not  exceeded  one-twentieth  of 
an  inch.  But  among  the  French  much  greater  power  of  chemically 
affecting  the  walls  is  attributed  to  ^intrusions,  and  in  instances  it 
certainly  seems  as  if,  in  addition  to  the  fluorine  and  boron  that 
we  all  know  penetrate  into  wall  rocks  during  the  escape  of  min- 
eralizers,  hydro-fluosilicic  acid  might  impart  silica  and  that  some 
of  the  bases,  and  especially  the  alkalies,  might  migrate  in  heated 
solutions,  to  a  moderate  distance. 

6.  Notwithstanding  the  truth  of  the  foregoing  generalities,  it  is 
a  curious  fact  that  contact  effects  are  sometimes  strangely  lacking 
where  we  would  naturally  expect  them,  and  that  they  are  often  of 
varying  intensity  and  irregular  distribution,  where  they  do  occur. 
These  anomalies  can  in  part  be  explained  by  the  general  principles 
already  cited,  of  which  no  doubt  the  presence  or  absence  of  min- 
eralizers,  the  superheated  or  relatively  cold  condition  of  the  intru- 
sion are  chief.  But  every  observer  of  wide  experience  is  sometimes 
much  puzzled  by  what  he  meets  in  Nature. 


I.  THE  ROCKS  PRODUCED  BY  CONTACT  METAMORPHISM. 

Although  the  principal  results  of  contact  metamorphism  are 
manifested  in  the  walls  of  the  intrusion,  the  igneous  rock  is  itself 
influenced.  It  is  therefore  necessary  to  note  both  the  internal  and 
the  external  effects,  or  those  upon  the  intrusion  and  those  upon  the 
walls.  The  area  over  which  the  latter  are  manifested  is  often 
called  the  aureole,  and  the  concentric  rings  of  decreasing  alteration 
as  one  passes  outward  from  the  intrusion  are  called  zones. 

Internal  Effects.  The  igneous  rock  suffers  a  relatively  rapid  loss 
of  heat  in  its  marginal  portions  as  compared  with  its  interior,  and 
as  a  result  it  very  commonly  assumes  a  porphyritic,  felsitic  or  even, 
just  at  the  contact,  a  glassy  texture,  although  it  may  be  granitoid 
within.  Where  these  textures  are  well  developed  the  passage 
from  one  to  the  other  is  extremely  gradual,  and  if  the  wall  rock 
has  been  originally  a  shale  or  a  clay  that  has  been  baked  to  a 


88  A  HANDBOOK  OF  ROCKS. 

dense  mass,  one  may  need  microscopic  examination  to  determine 
where  the  intrusion  ends  and  the  wall  rock  begins.  The  changes 
in  texture  in  the  intrusion  are  accompanied  more  or  less  by 
changes  in  chemical  composition  and  in  not  a  few  cases  progres- 
sive analyses  have  shown  the  margins  to  be  much  more  basic  than 
the  interior  of  the  intrusion.  The  chilling  of  the  former  has  thus 
produced  chemical  rearrangements  in  the  magma  previous  to  con- 
solidation. 

External  Effects.  Recalling  the  statement  earlier  made  that 
within  the  limits  already  set  forth  the  character  of  the  intrusion  is 
immaterial,  the  most  convenient  and  intelligible  method  of  treat- 
ment will  be  to  briefly  outline  several  typical  cases  wherein  the 
commoner  sedimentary  rocks  are  known  to  have  been  affected, 
and  then  to  refer  to  one  or  two  instances  wherein  igneous  or  re- 
gionally metamorphic  ones  have  suffered  alteration.  The  same 
order  will  be  preserved  for  the  sediments  as  appears  under  Chapters 
VII  and  VIII. 

Breccias  are  too  limited  in  distribution  to  be  a  serious  factor. 
Conglomerates  and  sandstones  so  generally  consist  of  silica,  that 
they  supply  but  little  raw  materials  of  a  favorable  kind.  The  small 
amounts  of  alumina  present  may  combine  with  the  silica  to  afford 
sillimanite  (A12O3  ,SiO2 )  and  stimulated  circulations  of  hot  water 
may  cause  added  deposition  of  quartz  around  the  grains5so  as  to 
develop  increased  hardness. 

With  shales  and  clay  rocks,  even  if  in  the  form  of  slate  (see 
later  p.  '$3=5)  the  effects  are  more  pronounced;  and  around  in- 
trusions in  them  well-marked  and  well  identified  zones  have  been 
described. 

At  the  contact  of  the  igneous  rock  with  the  sediments  a  breccia 
or  "  mixed  zone  "  of  intrusive  and  fragments  of  wall  rock  is  some- 
times, although  not  always,  met.  More  commonly  the  shales, 
slates,  clay  or  their  kindred  rocks  are  baked  and  altered  to  a 
dense,  flinty  product  known  as  a  hornfels  or  hornstone,  which 
latter  name  in  this  sense,  is  however,  not  to  be  confused  with  its 
use  for  flints  and  cherts.  It  breaks  in  irregular,  angular  masses 
and  has  a  very  close  resemblance  to  dense  trap.  Its  mineralogy 
is,  as  a  general  thing,  a  subject  for  microscopic  study,  but  it  may 
be  said  that  biotite  in  small  scales  is  rather  the  most  widespread 
mineral  present,  and  that  andalusite,  garnet,  cyanite,  staurolite, 
tourmaline,  ottrelite,  rutile,  hornblende,  feldspars  and  other  min- 


THE  METAMORPHIC  ROCKS.     INTRODUCTION.     89 

erals  more  or  less  characteristic  of  such  surroundings  frequently 
appear.  They  may  be  of  considerable  size  and  the  prisms  of 
andalusite  of  the  variety  chiastolite  with  the  light  and  dark  mal- 
tese  crosses  showing  in  their  cross-sections,  are  especially  frequent. 
As  the  contact  is  left  the  hornfels  often  passes  into  mica  schist. 
Farther  out  the  mineralogical  changes  become  less  marked ;  the 
andalusite  and  other  crystals  are  less  and  less  well  developed  and 
finally  shade  into  mere  dark  spots  or  aggregates  of  biotite,  mag- 
netite and  bituminous  matter.  When  even  these  fade  out  the  un- 
changed sediment  is  met.  In  some  localities  it  has  therefore  been 
possible  to  establish  three  zones,  which  are  in  the  reverse  order  of 
the  above  succession,  the  knotty  or  spotted  slates,  the  knotty 
mica  schists,  and  the  hornfels,  usually  with  andalusite.  By  knotty 
is  meant  the  aspect  given  by  the  larger  contact  minerals  in  the 
midst  of  finer  aggregates. 

These  are  the  names  adopted  for  a  well  known  contact  studied 
by  the  eminent  German  petrographer,  Rosenbusch,  in  the  Vosges 
Mountains.  At  a  famous  American  locality  in  the  Crawford  Notch 
of  the  White  Mountains,  on  the  slopes  of  Mt.  Willard  and  not  far 
from  the  Crawford  House,  the  granite  has  penetrated  an  argillitic 
mica  schist  or  micaceous  slate,  and  the  zones  are  somewhat  differ- 
ent. G.  W.  Hawes  in  1881  established  the  following  seven: 
I.  The  argillitic  mica  schist  (chloritic);  2.  Mica  schist  (biotitic) ; 
3.  Tourmaline  hornstone ;  4.  Tourmaline  veinstone  (a  small  con- 
tact band,  rich  in  tourmaline);  5.  Mixed  schists  and  granite; 
6.  Granite  porphyry  (biotitic) ;  7.  Granite  (hornblendic).  This  is 
one  of  the  most  complete  and  well-exposed  contacts  known,  and 
illustrates  both  external  and  internal  effects.  *  The  succession 
illustrates  the  alteration  of  chlorite  to  biotite  by  the  granite,  and 
then  near  the  contact  the  development  of  tourmaline  from  the 
boracic  and  fluoric  emanations  which  were  afforded  by  it.  On 
the  southeast  corner  of  Conanicut  Island,  in  Narragansett  Bay, 
granite  has  penetrated  Carboniferous  shales,  as  described  by  L. 
V.  Pirsson,f  and  has  baked  them  to  compact  hornfels  near 
the  contact.  Spotted  slates  are  likewise  met  resembling  those  de- 
scribed above.  Immediately  beneath  the  diabase  of  the  Palisade 
ridge  at  Hoboken,  N.  J.,  the  Triassic  shales  are  baked  to  a  compact 

*  Hawes'  paper  is  in  the  American  Journal  of  Science,  January,  1881,  p.  21. 
f  L.  V.  Pirsson,  On    the  Geology   and  Petrography  of  Conanicut   Island,  R.   I. 
Amer.  Jour,  of  Set.,  Nov.,  1893,  P-  3^3- 


90  A  HANDBOOK  OF  ROCKS. 

hornsfels  with  abundant  tourmalines,  and  near  Beemerville,  N.  J.,* 
a  great  dike  of  nepheline-syenite  has  come  up  through  Ordovi- 
cian  shales  and  has  altered  them  in  places  to  remarkably  dense 
black  hornsfels.  Near  Crugers,  on  the  Hudson  River,  mica- 
diorites  have  penetrated  mica  schists  and  have  developed  in  them 
a  considerable  number  of  characteristic  contact  minerals,  but  the 
changes  in  the  schists  are  not  specially  apparent  to  the  eye.f  As 
western  and  other  eastern  areas  are  further  studied,  no  doubt  addi- 
tional cases  will  be  fully  described.  Many  are  known  and  await 
careful  field  work. 

The  contact  effects  on  limestones  furnish  extremely  interesting 
phenomena  and  a  series  of  minerals  somewhat  different  from  those 
just  described.  On  account  of  the  general  lack  of  migration  of 
material  the  elements  of  the  minerals  must  be  present  in  the  unal- 
tered rock.  Pure  limestones  therefore  merely  crystallize  into  equally 
pure  marbles.  Siliceous  and  argillaceous  ones  become  thickly 
charged  with  biotite,  garnet,  vesuvianite,  scapolite,  pyroxenes  and 
amphiboles,  tourmaline,  spinel,  and  not  a  few  more.  Garnet  and 
vesuvianite  are  especially  characteristic.  Good  contacts  have  been 
met  at  several  American  localities.  Near  St.  John,  N.  B.,J  granite 
has  penetrated  Laurentian  limestone  and  has  developed  a  garnet 
zone,  with  more  or  less  pyroxene.  Diorites  cutting  or  including 
limestone  in  the  Cortland  series§  have  caused  the  formation  of 
pyroxene,  scapolite,  hornblende  and  other  minerals. 

In  the  valley  extending  from  Warwick,  N.  Y.,  southwest  to 
Sparta,  N.  J.,  are  most  instructive  exhibitions,  and  rich  mineral 
localities  are  based  on  them.  Granite  is  the  principal  intrusive.|| 
The  western  Adirondack  region  of  New  York  contains  many  more 
where  gabbro  and  limestone  come  together,  and  where  the  well 
known  mineral  localities  occur.  C.  H.  Smyth,  Jr.,  has  lately  iden- 
tified their  contact  nature  and  will  in  time  describe  them.  Abroad, 
the  region  about  Christiania  in  Norway  has  proved  to  be  classic 
ground  for  these  phenomena,  and  a  great  contact  of  diorite  on 
Triassic  limestone  at  Predazzo  in  the  Tyrolese  Alps  has  produced 
the  characteristic  zones  on  a  grand  scale. 

The  inclusions  of  wall  rock  caught  up  by  an  advancing  intru- 

*  J.  F.  Kemp.     Trans.  New   York  Acad.  Sci.,  xi.  p.  60. 

f  G.  H.  Williams.     Amer.  Jour.  Sci.,  Oct.,  1888,  p.  265. 

JW.  D.  Matthew,  Trans.  N.  Y.  Acad.  Sci.  XIII ,  194. 

§G.  H.  Williams,  Amer.  Jour.  Sci.,  Oct.,  1888,  267. 

1J.  F.  Kemp  and  Arthur  Hollick  Annals  N.  Y.  Acad.  Sci.  VII.,  644. 


THE  METAMORPHIC  ROCKS.     INTRO D  UCTION.    9 1 

sion  on  its  way  to  the  surface  are  instructive  examples,  and  often 
are  afterwards  found  entombed  in  the  igneous  rock  and  more  or 
less  altered.  The  lava  flows  of  Vesuvius  and  the  ejected  bombs 
have  been  of  extraordinary  interest  in  this  respect.  Limestones 
are  frequent  among  them  and  they  exhibit  the  same  zones  as  the 
larger  occurrences.  Vesuviamte,  in  fact,  received  its  name  from 
this  association. 

Of  the  remaining  members  of  the  grand  division  of  the  Aqueous 
rocks,  the  Carbonaceous  are  the  principal  ones  deserving  mention. 
Coal  seams  of  the  normal  bituminous  variety  have  been  cut  in  not  a 
few  places  by  igneous  dikes,  and  display  in  a  marked  degree  the 
metamorphosing  effect.  The  volatile  hydrocarbons  have  been 
driven  off  and  the  coal  has  become  an  impure  coke.  The  Triassic 
coal  basins  of  Virginia  and  North  Carolina  exhibit  many  instances 
where  diabase  dikes  have  wrought  the  change,  and  in  the  region  of 
Puget  Sound  basalt  intrusions  have  effected  similar  results.  In 
Colorado  and  New  Mexico,  the  near  approach  of  an  igneous  sheet 
has  brought  about  the  formation  of  anthracite,  and  in  fact  all 
grades  of  coal  can  be  detected  from  rich  bituminous  to  hard  anthra- 
cite, according  to  the  nearness  of  the  dike  or  laccolite. 

Reference  may  also  be  made  to  the  hills  of  soft  magnetite,  near 
Cornwall,  Penn.,  where  a  great  dike  of  diabase  has  altered 
limonite  to  this  more  crystalline  and  thoroughly  anhydrous 
mineral. 

Where  intrusions  cut  other  igneous  or  metamorphic  rocks  the 
effects  are  much  less  apparent,  because  the  walls  are  resistant  to 
change,  being  themselves  already  crystalline.  Around  granites, 
however,  even  in  these  conditions,  great  pegmatite  dikes  and  veins 
are  copiously  produced,  which  seems  to  be  in  large  part  brought 
about  by  escaping  heated  vapors  and  solutions. 

Remarkable  cases  of  contact  metamorphism  are,  however,  cer- 
tainly caused  by  these  last  named  agents.  As  rocks  they  are  not 
specially  abundant,  although  of  great  scientific  interest.  Some  in- 
trusions have  emitted  copious  emanations  of  hydrofluoric  and  bor- 
acic  acid  in  conjunction  with  superheated  steam.  These  vigorous 
reagents  have  attacked  the  wall  rocks,  when  originally  formed  of 
crystalline  silicates,  making  them  porous  and  cellular  from  the  de- 
struction of  feldspars,  and  have  often  caused  the  crystallization  of 
quartz,  tourmaline,  topaz,  fluoric  micas,  fluorite,  apatite  and  other 
characteristic  minerals  of  which  cassiterite  is  of  much  economic 


92  A  HANDBOOK  OF  ROCKS. 

importance.  Such  metamorphic  products  when  essentially  consist- 
ing of  quartz  and  mica  are  called  greisen.  Tourmaline  granites 
likewise  result.  It  is  not  to  be  overlooked,  however,  that  mineral- 
izers  have  also  played  a  large  part  in  the  cases  earlier  cited,  nor 
should  the  remark  be  omitted  in  conclusion  that  they  and  similar 
agents  have  been  of  vast  practical  importance  in  the  formation  of 
ores. 


CHAPTER  X. 

THE  METAMORPHIC  ROCKS,  CONTINUED.     THE  ROCKS  PRODUCED 

BY  REGIONAL  METAMORPHISM.     INTRODUCTION.     THE 

GNEISSES  AND  CRYSTALLINE  SCHISTS. 

INTRODUCTION. 

This  subdivision  embraces  rocks  that  differ  widely  among  them- 
selves, but  that  have  nevertheless  important  features  in  common. 
The  following  generalities  are  applicable  in  a  large  way  and  will 
serve  to  emphasize  some  of  the  most  important  points. 

1.  Regionally  metamorphosed  rocks  are  all  more  or  less  per- 
fectly crystalline.     This  is  least  developed  in  the  slates. 

2.  They  are  all  more  or  less  decidedly  laminated  or  foliated,  al- 
though some  amphibolites,  marbles  and  serpentines  are  quite  mas- 
sive.    The  laminations  are  due  to  the  arrangement  of  the  con- 
stituent minerals,  and  especially  the  dark-colored  ones,  in  parallel 
alignment,  so  that  light  and  dark  layers  stand  out  conspicuously. 
The  terms  bedded  and  stratified  should  never  be  applied  to  them 
because  the  banding  is  largely  due  to  dynamical  processes,  and 
has  no  necessary  connection  with  original  sedimentation. 

3.  They  are  of  ancient  geological  age  or  else  are  in  greatly  dis- 
turbed districts. 

It  is  important  in  connection  with  these  rocks  to  distinguish  be- 
tween the  effects  produced  by  heat  or  thermal  metamorphism  and 
the  effects  produced  by  mechanical  forces  or  dynamic  meta- 
morphism. By  thermal  metamorphism  we  understand  the  alter- 
ations caused  by  heat  not  necessarily  accompanied  by  the  mechani- 
cal effects  such  as  shearing,  crushing  and  the  like,  that  are 
comprehended  under  dynamic  metamorphism.  Contact  metamor- 
phism is  of  course  a  variety  of  the  former  which,  however,  is  also 
brought  into  play  alike  when  rocks  are  so  deeply  buried  that  they 

93 


94  A  HANDBOOK  OF  ROCKS. 

come  within  the  sphere  of  influence  of  the  earth's  interior  heat,  and 
when  from  dynamic  stresses,  they  are  crushed  so  that  their  particles 
move  or  slide  under  great  pressure  on  one  another  and  develop 
heat  by  friction.  If  we  imagine  for  a  moment  great  bodies  of 
rocks  which  have  definite  crushing  resistances,  buried  under  a  load 
of  overlying  strata  so  deep  within  the  earth  that  their  limits  of  re- 
sistance are  exceeded,  yet  so  confined  that  they  cannot  fly  apart, 
we  perceive  that  they  must  yield  by  internal  crushing,  and  if  the 
upheaval  of  a  mountain  range  eases  the  strain,  that  they  must  flow 
in  a  mass.  It  is  to  this  flow,  accompanied  by  shearing,  that  the 
lamination  of  metamorphic  rocks  is  largely  due.  Prominent  or 
conspicuous  minerals  are  strung  out  in  parallel  alignment,  often- 
times with  wavy  folds  and  curves,  and  in  the  end  a  foliated  or 
laminated  structure  is  superinduced  that  suggested  the  bedding  of 
sediments  to  the  early  geologists.  It  is  not  to  be  denied,  however, 
that  the  laminations  do  at  times  correspond  to  original  bedding, 
because  where  the  contrasts  in  chemical  and  mineralogical  compo- 
sition, among  the  layers  are  pronounced,  they  doubtless -mark  such 
correspondence,  but  cases  are  well  known  of  old  conglomerate 
beds  passing  directly  across  the  prevailing  schistosity  of  a  gneissic 
district. 

During  these  shearing  and  flow  movements  larger  crystals,  such 
as  the  feldspars  of  porphyries,  and  the  larger  uncrushed  nuclei  of 
minerals  in  a  general  pulp  are  squeezed  and  stretched  into  lenses, 
and  remain  like  eyes  between  eyebrows,  so  that  they  are  called 
"Augen  "  from  the  German  word  for  eyes.  Swirling  curves  and 
eddies  in  the  laminations  are  also  familiar  phenomena  and  cannot 
be  explained  in  any  other  way. 

These  changes  may  take  place  without  mineralogical  alteration, 
as  when  granitoid  rocks  pass  into  gneisses  that  contain  simply  the 
crushed  fragments  of  the  originals,  but  as  a  general  thing  new  com- 
binations are  formed  in  the  metamorphosed  rock.  Pyroxene  passes 
into  hornblende;  soda-lime  feldspars  become  scapolite  or  saussurite, 
and  other  changes  ensue  that  are  best  detected  with  the  microscope. 
Sedimentary  rocks  suffer  entire  recrystallization,  and  sometimes  so 
thoroughly  lose  their  original  characters  that  no  clue  is  afforded  as 
to  their  history.  In  regional  metamorphism  precisely  as  in  the 
case  of  the  contact  metamorphic  rocks,  it  is  generally  believed  that 
there  is  no  change  in  composition,  except  perhaps  by  the  loss  of 
volatilizable  ingredients,  but  only  rearrangement  of  elements.  A 


THE  METAMORPHIC  ROCKS.     INTRODUCTION.  '  95 

gross  analysis  of  a  reasonably  large  sample  will  therefore  give  a 
clue  to  the  composition  of  the  original.  Heated  waters,  generally 
charged  with  mineral  matter  and  steam  have  no  doubt  contributed 
largely  in  bringing  about  the  final  result. 

The  Regionally  Metamorphosed  rocks  will  be  described  under 
the  following  heads : 

1.  The  Gneisses  and  Crystalline  Schists. 

2.  The  Quartzites  and  Slates. 

3.  The    Crystalline  Limestones  and  Dolomites :    The  Ophical- 
cites,  Serpentines  and  Soapstones. 

THE  GNEISSES. 

Introductory. — Gneiss  is  an  old  word,  that  originated  among  the 
early  German  miners  in  the  Saxon  districts.  It  was  especially  ap- 
plied by  them  to  laminated  rocks  of  the  mineralogical  composition 
of  granite,  and  in  this  sense  it  is  quite  widely  employed  to-day .* 
But  there  are  many  important  gneisses  that  correspond  in  min- 
eralogy to  the  other  plutonic  rocks,  and  that  are  quite  as  properly 
designated  by  this  name,  so  that  gneiss  has  come  to  be  a  term  that 
is  of  loose  geological  significance  very  much  as  is  trap,  but  that 
is  none  the  less  useful  for  this  reason.  We  may  therefore  define  _ 
gneiss  as  a  laminated,  metamorphic  rock  that  usually  corresponds 
in  mineralogy  to  some  oncfof  the  plutonic  types.  Gneisses  differ 
from  schists  in  the  coarseness  of  the  laminations,  but  as  these  be- 
come finer  they  pass  into  schists  by  insensible  gradations. 
Varieties  are  sometimes  indicated  by  prefixing  the  name  of  the 
most  prominent  silicate,  usually  one  of  the  ferro-magnesian  group, 
thus  hornblende-gneiss,  biotite-gneiss,  pyroxene-gneiss,  but  we  also 
often  speak  of  quartz-gneiss,  orthoclase-gneiss,  plagioclase-gneiss, 
garnet-gneiss  and  the  like. 

It  is  evident  at  once  that  the  above  names  are  incomplete. 
Hornblende-gneiss,  for  instance,  does  not  indicate  whether  the 
rock  contains  orthoclase  or  plagioclase,  quartz  or  no  quartz,  and 
the  other  ones  cited  are  open  to  the  same  or  similar  objections,  and 
if  in  the  endeavor  to  embody  fuller  descriptions  we  string  together 
the  names  of  all  the  minerals  in  the  rock,  we  employ  an  objection- 
able and  awkward  method  of  coining  names.  A  system  has,  how- 
ever, been  suggested  by  C.  H.  Gordon,*  in  a  recent  paper  that 
obviates  many  of  these  objections  and  that  is  adopted  below  with 

*Bulletin  of  the  Geological  Society  of  America,  VII.,  122. 


96 


A  HANDBOOK  OF  ROCKS. 


some  abbreviations  suitable  for  an  elementary  book.  It  is  based 
on  the  parallelism  that  exists  between  the  mineralogy  of  gneisses 
and  that  of  the  massive  plutonic  rocks,  and  it  avails  itself  of  the 
short  names  of  the  latter,  that  indicate  in  each  case,  a  definite 
combination  of  minerals  to  describe  the  aggregates  present  in  the 
former.  Two  sedimentary  terms  are  also  added. 


Massive  Type. 

Gneiss  of  Correspond- 
ing Mineralogy. 

Sedimentary 
Type. 

Derived  Gneiss. 

Granite 
Syenite 
Diorite 
Gabbro 
Pyroxenite 
Peridotite 

Granitic  Gneiss 
Syenitic  Gneiss 
Dioritic  Gneiss 
Gabbroic  Gneiss 
Pyroxenitic  Gneiss 
Peridotitic  Gneiss 

Conglomerate 
Sandstone 

Conglomerate  Gneiss 
Quartzite  Gneiss 

Dr.  Gordon  also  suggests  that  when  gneisses  are  evidently  dy- 
namic derivatives  from  a  massive  rock,  that  this  relationship  be 
indicated  by  using  the  terms  granite-gneiss,  syenite-gneiss  and  so 
on.  If,  however,  differentiations  in  the  magma  before  crystallizing 
have  given  rise  to  laminations,  that  such  be  distinguished  by  the 
adjective  gneissoid,  as  gneissoid  gabbros. 

Gneisses  are  occasionally  met  which  do  not  exactly  correspond 
to  any  of  the  above  names.  Chlorite,  for  example,  is  a  not  un- 
common mineral,  and  while  it  is  evidently  an  alteration  product 
from  pyroxene,  hornblende  or  biotite,the  original  mineral  is  not  at 
once  apparent,  and  some  such  name  as  chlorite-gneiss  must  be 
used.  In  the  same  way  cordierite-gneiss  describes  those  rare 
varieties  containing  cordierite  (iolite  and  dichroite  are  synonyms 
of  cordierite);  sillimanite-gneiss,  garnet-gneiss,  epidote-gneiss  and 
others  convey  in  their  names  their  characteristic  features. 


ANALYSES  OF  GNEISSES. 

Chemical  analyses'1  often  enable  us  to  trace  back  gneisses  to  their 
original  rocks,  whether  igneous  or  sedimentary,  but  it  requires 
careful  study  of  correct  type  analyses  and  some  familiarity  with 
their  ranges  in  composition  to  do  it.  So  far  as  their  number 
admits  the  analyses  quoted  on  earlier  pages  will  be  found 
suggestive : 


THE  GNEISSES.  97 


Si02. 

Al,03. 

Fe203. 

FeO. 

CaO. 

MgO. 

K2O. 

Na2O.     Loss  or  H2O. 

I. 

76.61 

1245 

'•33 

0.84 

5-42 

3.12 

0.53 

2. 

74-95 

9.42 

7-47 

.    . 

1.65 

0.13 

2.O2 

4.05 

I.O2 

3- 

73-47 

X5  °7 

[.15 

4.48 

0.  12 

038 

5-59 

. 

4- 

71.46 

15.06 

.'  . 

243 

1.40 

0.42 

5-'7 

3-23 

0.83 

5- 

69.35 

18.83 

2.OO 

5-94 

3-78 

6. 

6994 

14.85 

7.62 

2.10 

0.97 

4-33 

4-3° 

0.70 

7- 

61.96 

'9-73 

4.60 

0.35 

1.81 

2.50 

0.79 

1.82 

8, 

57.66 

22.83 

7-74 

1.16 

3.56 

5-72 

0.60 

I.50 

9- 

57.20 

*957 

952 

0.59 

5-73 

4.40 

0.28 

2.13 

0.88 

10. 

54.89 

!3-67 

1-35 

5.63 

4.70 

8-34 

'•95 

2.76 

I.  Granitic  gneiss,  west  side  of  Black  Hills  4oth  Par.  Survey,  I.  p.  no.  R.  W. 
Woodward,  Anal.  2.  Called  a  dioritic  gneiss  in  reference,  contains  hornblende, 
quartz,  plagioclase,  orthoclase.  Idem.:  R.  W.  Bunsen,  Anal.  3.  Conglomerate  gneiss, 
so-called  granite;  Munson,  Mass.  Quoted  by  G.  P.  Merrill,  Stones  for  Building  and 
Decoration  p.  418.  4.  Granitic  gneiss,  Iron  Mountain,  Wyo.  R.  W.  Woodward, 
Anal.  See  under  No.  I.  5.  Dai'k  variety  of  No.  3.  6.  Granite  gneiss,  derived 
from  a  hornblende  granite,  Trembling  Mountain,  Quebec,  Fundamental  Laurentian 
of  Logan.  F.  D.  Adams,  Amer.  Jour.  Sci.t  July,  1895,  ?•  ^7-  W.  C.  Adams,  Anal. 
7.  Quartzitic  gneiss,  with  garnet,  sillimanite,  graphite  and  pyrite;  St.  Jean  de  Matha, 
Quebec.  Idem.  N.  N.  Evans,  Anal.  8.  Granitic  gneiss,  probably  a  metamorphosed 
clay  or  slate.  Trembling  Lake,  Quebec.  Contains  garnets  and  sillimanite.  F.  D. 
Adams,  Amer.  Jour.  Set.,  July,  1895,  P-  ^7-  W.  C.  Adams,  Anal.  9.  Dioritic 
gneiss,  New  York  City.  P.  Schweitzer.  Amer.  Chemist,  VI.  457,  1876.  10.  Gneiss 
containing  malacolite,  scapolite,  orthoclase,  graphite,  pyrite.  Rawdon,  Quebec. 
See  under  No.  8. 

Comments  on  the  Analyses.  Nos.  I,  4  and  6  are  clearly  derived 
from  granites,  presumably  by  dynamic  metamorphism.  The  an- 
alyses correspond  closely  in  their  general  features  with  those 
given  on  p.  30  except  that  the  A12O3  of  No.  I  is  a  trifle  low,  and 
the  Fe2O3  of  No.  6  a  trifle  high.  Nos.  3  and  5  are  now  known 
to  be  metamorphosed  Cambrian  conglomerate,  although  so  thor- 
oughly recrystallized  as  to  be  a  well  known  commercial  granite. 
The  conglomerate  must  have  come  from  granitic  or  dioritic  orig- 
inal rocks.  Nos.  7  and  8  correspond  to  the  analyses  of  slates  as 
noted  by  F.  D.  Adams  in  the  original  reference  (see  also  under 
slates,  p.  107).  No.  10  as  noted  by  Adams  is  of  doubtful  interpreta- 
tion. The  high  alkalies,  lime,  magnesia  and  the  moderate  silica 
suggest  a  basic  syenite  or  trachyte,  but  the  alumina  is  exception- 
ally low  for  these.  It  may  be  a  tuff  or  a  slightly  altered  sediment 
from  these  originals.  No.  2  is  a  very  anomalous  rock,  and  it  is 
difficult  to  refer  it  to  an  original  diorite,  it  is  so  high  in  silica  and 
so  low  in  alumina.  The  iron  is  very  large  for  so  acidic  a  rock. 
No.  9  is  undoubtedly  an  altered  sediment  as  indicated  by  the  local 
geology.  Notwithstanding  the  anomalies  of  composition,  chem- 
7 


98  A  HANDBOOK  OF  ROCKS. 

ical  analyses  supply  one  of  the  surest  clues  to  the  geological  his- 
tory of  gneisses  and  it  is  to  be  hoped  that  they  will  be  multiplied 
in  America.  At  present  but  few  are  available,  far  fewer  than  of 
igneous  rocks. 

Alteration.  The  alteration  of  gneisses  is  similar  in  all  respects  to 
that  of  their  corresponding  massive  types.  The  feldspars  alter  to 
kaolin,  the  micas  and  hornblende  to  chlorite  and  the  rock  softens 
down  to  loose  aggregates  that  contribute  heavily  to  the  sedimen- 
tary rocks. 

Distribution.  Gneisses  are  abundant  in  ancient,  geological  form- 
ations. The  early  Archean  is  their  especial  home,  and  they  form 
the  largest  part  of  its  vast  areas  in  Canada,  around  the  Great  Lakes, 
along  the  Appalachians  and  in  the  Cordilleran  region.  But 
no  single  geological  age  monopolizes  them  any  more  than 
plutonic  rocks.  There  are  Cambrian  and  even  Carboniferous 
gneisses  in  New  England,  and  dynamic  metamorphism  may  pro- 
duce them  from  massive  rocks  of  almost  any  age.  The  later 
geological  formations  are,  however,  seldom  buried  sufficiently  deep 
to  be  in  favorable  situations.  Much  the  same  holds  true  of  Europe 
and  the  rest  of  the  world.  The  gray  and  red  gneisses  of  the  min- 
ing districts  about  Freiberg,  in  Saxony,  those  of  the  Highlands  of 
Scotland,  and  in  Scandinavia,  and  the  wonderful  exhibitions  of 
dynamic  metamorphism  in  the  Alps  are  to  be  cited  as  of  unusual 
historic  and  scientific  interest. 

Granulite.  Granulite  is  a  word  that  has  possessed  somewhat 
contrasted  meanings  according  as  it  has  been  used  in  Germany, 
France  or  England.  In  Germany  as  first  employed  it  was  applied 
to  a  finely  gneissoid  rock  that  consists  chiefly  of  feldspar,  quartz  and 
garnets.  These  original  granulites  have  other  minerals  more  or 
less  prominently  developed,  of  which  cyanite,  augite,  biotite  and 
hornblende  are  chief.  The  texture  of  the  rock  is  extremely  dense, 
and  except  for  the  garnets,  cyanite  or  augite  the  individual  min- 
erals are  hardly  discernible.  Among  French  and  English  speaking 
peoples  granulite  has  been  applied  to  granitic  rocks  that  appear  to 
the  eye  to  be  chiefly  quartz  and  feldspar,  although  the  microscope 
may  show  muscovite.  They  are  practically  binary  granites,  or 
rich  quartz  and  feldspar  gneisses.  The  name  has  also  been  used 
for  coarse  plutonic  rocks  that  have  been  crushed  down  by  dyna- 
mic metamorphism  into  a  finely  granular  and  homogeneous  aggre- 
gate. But  so  far  as  metamorphic  rocks  have  been  met  in  America, 


THE  MICA  SCHISTS.  99 

y 

cases  are  very  rare  which  cannot  be  satisfactory  described  without 
the  use  of  this  word,  that  has  been  so  perverted  from  its  original 
application  as  to  be  practically  valueless  without  an  accompanying 
explanation. 

THE  CRYSTALLINE  SCHISTS. 

The  crystalline  schists  have  finer  laminations  than  the  gneisses, 
but  in  other  respects  the  mineralogy  is  often  much  the  same,  and 
as  already  stated  no  very  sharp  line  can  be  drawn  between  them. 
It  is  important  to  note  that  the  words  "  schiste  "  of  the  French 
and  "Schiefer"  of  the  Germans  are  applied  to  shales,  slates  and 
metamorphic  schists  indiscriminately,  but  in  English  schist  is  only 
used  for  metamorphic  rocks.  The  more  important  schists  are 
broadly  classified  according  to  the  principal  ferro-magnesian  silicate 
that  is  present  into  the  following  three  groups  under  which 
they  will  be  taken  up. 

(a)  Mica  schists. 

(b)  Hornblende-schists  or  Amphibolites. 

(c)  Various  Minor  Schists. 

THE  MICA   SCHISTS. 


SiO2. 

MA- 

Fe203. 

FeO. 

CaO. 

MgO. 

K2O. 

Na2O. 

H2O. 

I. 

82.38 

11.84 

. 

2.28 

.  . 

1.  00 

0.83 

0.38 

0.77 

2. 

79-5° 

13-36 

2.87 

0.71 

0-95 

4-69 

0.36 

0.78 

3- 

69-45 

14.24 

.  . 

6-54 

2.66 

'•35 

2.52 

4.02 

0.52 

4- 

66.21 

1  8.  60 

.  . 

5-34 

0.44 

1.24 

3.80 

2.16 

2.04 

5- 

62.98 

16.88 

2.48 

5.00 

tr. 

1.58 

7-45 

3.02 

.  . 

6. 

61-57 

19-53 

5-44 

2.61 

tr. 

1.90 

2.14 

3-48 

.  . 

7- 

60.49 

19-35 

0.48 

5-98 

1.  08 

2.89 

3-44 

2-55 

3.66 

8. 

57.67 

17.92 

.  . 

9.00 

3-19 

329 

3-86 

1.09 

3-19 

9- 

55.12 

24.32 

6.13 

4-99 

tr. 

tr. 

2.83 

2.71 

.  . 

10. 

49.00 

23-65 

8.07 

0.63 

0.94 

9.11 

'•75 

3-4i 

I.  Mica-schist,  rich  in  quartz,  Monte  Rosa,  Switzerland.,  Zulkowsky,  Sitz.  Wiener 
Akad.  XXXIV.,  41,  1895.  2.  Mica-schist,  with  quartz  and  green  mica  Zermatt. 
Switzerland.  Bunsen  in  RotJi's  Tabellen,  1862.  3.  Garnetiferous  mica  schist  with 
feldspar,  Brixen.  Tyrol.  Schonfeld  and  Roscoe,  Ann.  der.  Chem.  u.  P/iar.,  XCI.,  1854. 
305.  4.  Mica-schist  near  Meissen,  Saxony,  Hilger  quoted  in  Roth's  Tabellen,  1879. 

5.  Mica-schist,  Crugers,  N.  Y.,  contains    quartz,  orthoclase,  biotite,  muscovite,  little 
oligoclase,  etc.     F.  L.  Nason  for  G.  H.  Williams,  Amer.  Jour.  Sci.t  Oct.,  1888.     259. 

6.  Crumpled    garnetiferous  mica-schists.    Idem.       7.    Argillitic   mica-schist,    G.    W. 
Hawes,  Geology  of  New  Hampshire,  Part  III.  219.     8.  Mica-schist  near  Messina, 
Sicily,  Ricciardi.  Quoted  in  Roth's  Tabellen,  1884,  p.  ix.       9.  Staurolite  mica-schist, 
with  biotite,  muscovite,   quartz,  sillimanite,  garnet.     See  under  No.  6.     10.   Sericite 
schist,  Wisconsin,  Wis.  Geol.  Surv.t  I.  304. 


ioo  A   HANDBOOK  OF  ROCKS. 

Comments  on  the  Analyses.  Like  the  majority  of  gneisses  the 
mica-schists  are  more  or  less  closely  parallel  with  the  granites  in 
chemical  composition  because  the  constituent  minerals  are  so 
largely  the  same  in  both.  But  where  they  have  been  formed  from 
metamorphosed  sediments  such  as  shales,  clays,  and  the  like, 
the  alkalies  are  often  lower  than  is  the  case  with  siliceous 
igneous  rocks,  and,  what  is  still  more  characteristic  of  sediments 
as  contrasted  with  highly  siliceous  igneous  rocks,  the  magnesia  is 
in  excess  of  the  lime.  A  comparison  of  the  above  analyses  with 
those  of  the  rhyolites,  trachytes,  granites  and  syenites  earlier  given 
will  forcibly  bring  this  out.  The  local  geology  as  well  as  the 
analyses,  indicate  that  there  is  little  doubt  that  Nos.  5,  6,  7  and  9 
are  altered  sediments,  and  the  presumption  is  strong  that  most  of 
the  others  are  also. 

Mineralogical  Composition.  Varieties.  The  most  prominent  and 
abundant  minerals  in  the  mica-schists  are  quartz,  muscovite  and 
biotite.  While  they  are  more  or  less  interleaved  together,  yet 
close  examination  of  the  coarser  varieties  shows  that  they  are  in. 
layers  irregularly  parallel  and  to  a  large  extent  distinct.  The 
minerals  are  in  all  degrees  of  relative  abundance,  quartz  sometimes 
largely  predominating  and  marking  a  passage  to  the  quartzites, 
while  again  the  micas  may  be  in  great  excess.  Both  muscovite 
and  biotite  are  met,  the  former  being,  perhaps,  rather  the  more 
abundant.  With  these  chief  minerals  are  almost  always  asso- 
ciated very  considerable  amounts  of  feldspar,  both  orthoclase  and 
plagioclase,  and  variable  proportions  of  garnet,  staurolite,  cyanite, 
sillimanite,  tourmaline,  apatite,  pyrite  and  magnetite. 

The  garnet  and  staurolite  may  exhibit  surprisingly  well  devel- 
oped crystals  and  illustrate  the  extraordinary  power  of  certain 
compounds  to  crystallize  under  circumstances  apparently  ill- 
adapted  to  their  perfect  development. 

Mica-schists  embrace  a  series  from  rather  coarsely  crystalline 
varieties  to  others  that  are  excessively  fine-grained  and  that  are 
near  relatives  of  the  slates.  The  minerals  of  the  latter  may  be  of 
microscopic  dimensions,  and  only  the  aggregate  of  shining  scales 
reveals  them  as  mica.  Such  aggregates,  of  a  silvery  white  color 
but  of  composition  essentially  the  same  as  normal  muscovite,  are 
called  sericite,  and  the  corresponding  schists,  sericite-schists.  A 
related  soda- mica  (muscovite  and  its  relatives  are  potash  micas) 
is  called  paragonite.  Hydromica  is  a  name  applied  many  years 


THE  AMPHIBOLITES.     THE  MINOR  SCHISTS.    101 

ago  by  Dana  to  sericite,  paragonite,  and  perhaps  others  resembling 
them,  so  that  for  these  finely  micaceous  schists,  especially  in  our 
eastern  states,  hydromica  schist  is  a  field  name  tfiat  is  largely  used 
in  practice  and  in  geological  reports.  These  fine-grained  mica- 
schists  that  approximate  slates  are  also  made  a  special  group  by 
many,  under  the  name  phyllite,  a  very  useful  term  and  one  to  be 
strongly  commended.  Mica-schists  are  also  met  that  are  high 
in  lime  and  that  mark  transitions  to  the  crystalline  limestones. 
The  abundance  of  calcite  or  dolomite  betrays  them,  and  to  such 
the  names  calcareous  schist  or  calc-schist  are  applied. 

Mica-schists  result  from  the  thorough  metamorphism  or  recrys- 
tallization  of  sandstones,  shales  and  clays,  and  also  from  the  crush- 
ing and  excessive  shearing  of  igneous  rocks,  granitoid  and  por- 
phyritic  alike.  A  possible  origin  from  ancient  volcanic  tuffs  is 
always  to  be  considered  in  the  study  of  a  district,  but  the  ques- 
tions of  origin  are  obscure  and  are  subjects  for  thorough  chemical 
and  microscopical  investigation. 

Alteration.  The  mica-schists  are  rather  resistant  to  alteration 
and  often  appear  on  mountain  tops.  When  alteration  does  pre- 
vail, they  soften  to  masses  of  quartz  sand,  chlorite  scales  and  kaolin. 

Distribution.  The  mica-schists  form  the  country  rock  over  vast 
areas  in  New  England  and  to  the  south  along  the  eastern  Appa- 
lachians. Although  long  regarded  as  of  uncertain  or  obscure  geo- 
logical relations  they  are  now  recognized  in  large  part  at  least  as 
metamorphosed  Cambrian  and  Ordovician  shales  or  related  sedi- 
ments. Around  Lake  Superior  and  in  the  regionally  metamor- 
phosed areas  of  the  West  they  are  not  lacking. 


THE  HORNBLENDE  SCHISTS  OR  AMPHIBOLITES. 

Introductory. — Under  dynamic  metamorphism  the  basic  igneous 
rocks  whose  chief  bisilicate  is  pyroxene,  pass  very  readily  into 
hornblendic  rocks,  with  a  greater  or  less  development  of  schistosity. 
On  account  of  the  prevailing  parallel  arrangement  of  the  prismatic 
crystals  of  hornblende,  schistosity  is  seldom  entirely  lacking,  but 
where  less  distinct  the  name  amphibolite  has  proved  to  be  a  use- 
ful alternative,  and  indeed  is  of  wide  general  application.  Sedimen- 
tary rocks  are  also  known  in  rarer  instances  to  yield  similar 
results. 


102  A  HANDBOOK  OF  ROCKS. 

SiO2.       A12O3.      Fe2O3.      FeO.       CaO.      MgO.      K2O.       Na2O. 


I. 

52.39 

16.13 

1.64 

1.44 

8.76 

4.70 

1.42 

2-59 

0.17 

2. 

5°-44 

8.18 

i.  06 

6.28 

11.55 

17-63 

0.50 

2.98 

0.98 

3- 

49.19 

18.71 

5-°3 

4.04 

5-92 

7.98 

0.77 

1.44 

5-°5 

4. 

46.31 

11.14 

21.69 

9.68 

tr. 

6.91 

4-44 

5- 

44-49 

16.37 

507 

5-5° 

7-94 

7.50 

0.56 

2-59 

4.99 

I.  Hornblende-schist,  Grand  Rapids,  Wis.  Geology  of  Wis.  IV.  629.  Also,  Fe  in 
pyrite,  0.34;  8,0.39;  P2O5,  0.28  Cain  apatite  0.815.  2.  Pseudo-diorite  of  Becker, 
Knoxville,  Calif.  Monograph  X1IL  U.  S.  Geol.  Surv.  101,  W.  H.  Melville,  Anal. 
Also,  MnO  0.213,  Cr2O3  0.480.  3.  Hornblende-schist  derived  from  gabbro,  Lower 
Quinnesec  Basin,  Wis.  R.  B.  Riggs  for  G.  H.  Williams,  Bull.  62  U.  S.  Geol.  Surv. 
p.  89.  Also  CO2  1.82.  4.  Hornblende-schist  near  Cleveland  Mine,  Mich.  Foster  and 
Whitney  Rept.  on  the  Iron  Lands  of  Lake  Superior,  p.  92.  5.  Hornblende-schist, 
Lower  Quinnesec  Falls,  Wis.  R.  B.  Riggs  for  G.  H.  Williams,  Bull.  62.  U.  S.  Geol. 
Surv.  p.  91.  Also  CO2  5.38. 

Comments  on  the  Analyses.  The  analyses  indicate  basic  rocks,  of 
decidedly  variable  composition.  Nos.  3  and  5  are  certainly  sheared 
igneous  rocks.  No.  2  is  regarded  by  Becker  as  a  metamorphosed 
sediment.  It  is  quite  different  from  the  others  in  its  low  alumina, 
and  its  great  excess  of  magnesia  over  lime.  No.  I  appears  to  be 
an  altered  igneous  rock  and  No.  4  is  probably  the  same.  Aside 
from  exhibiting  the  composition  of  these  rocks,  the  analyses  are 
interesting  when  compared  with  those  of  the  basic  diorites  (p.  47) 
and  the  gabbros  £nd'pyroxenites  (p.  49.) 

'Mincrdlogictil  Co',nposition.  Varieties.  The  most  abundant  min- 
'eral'ih  these  rocks  is' naturally  hornblende.  With  it  are  associated 
oftentimes  biotite,augite,  plagioclase,  garnet,  magnetite,  pyrite  and 
pyrrhotite ;  but  quartz  except  as  forming  veinlets  is  not  often  met 
nor  is  it  to  be  expected  in  such  basic  rocks.  The  commonest  va- 
riety of  hornblende  is  black  to  the  eye  but  is  green  in  thin  section. 
It  forms  prismatic  crystals  from  moderately  coarse  to  microscopic- 
ally fine.  The  prisms  are  interlaced  so  as  to  make  a  very  tough 
aggregate  and  one  that  breaks  with  difficulty  under  the  hammer. 
Light  green  actinolite  may  also  form  schists.  Black  scales  of  bio- 
tite  appear  interlaminated  with  the  hornblende.  The  augite  is  not 
readily  distinguished  from  the  hornblende  with  the  eye  alone.  It 
is  in  large  degree  the  remnants  of  original  pyroxenes  that  have 
partially  passed  into  hornblende  during  the  metamorphic  process. 
The  plagioclase  also  represents  to  a  great  extent  the  feldspar 
that  was  in  the  original  gabbro  or  other  igneous  rock  from  which 
the  amphibolite  has  been  derived.  The  plagioclase  is  often  re- 
placed by  secondary  products,  such  as  epidote,  calcite,  scapolite 


THE  AMPHIBOLITES.     THE  MINOR  SCHISTS.  103 

and  others,  which  together  make  up  the  aggregate  formerly  called 
saussurite  and  regarded  as  an  individual  mineral.  The  minor 
accessories  magnetite,  pyrite,  pyrrhotite  and  garnet  deserve  no 
special  mention.  Except  magnetite,  which  never  fails,  they  are  of 
more  or  less  irregular  occurrence. 

Alteration.  The  hornblende  passes  readily  into  chlorite  and 
softens  to  a  scaly  mass  with  the  separation  of  much  limonite 
that  yields  a  characteristic,  rusty  outcrop.  If  any  pyrite  or  pyrrho- 
tite is  present  it  greatly  expedites  the  alteration  by  its  contri- 
bution of  sulphuric  acid.  The  feldspars  yield  calcite  and  kaolin 
and  the  whole  mass  becomes  a  rusty  clay  or  soil. 

Occurrence.  The  hornblende-schists  constitute  individual  belts  in 
schistose  regions  in  the  midst  of  other  metamorphic  rocks  and  also 
great  areas  by  themselves.  Dikes  and  sheets  of  diabase  and  plu- 
tonic  masses  of  gabbro  in  districts  that  have  been  subjected  to 
violent  dynamic  upheavals  readily  pass  into  them.  The  same 
areas  in  the  Eastern  States  that  were  cited  under  gabbro  contain 
them,  and  they  are  minor  members  in  the  schistose  districts  of 
New  England.  Around  Lake  Superior  they  form  a  most  impor- 
tant part  of  the  geology  of  the  iron  ore  regions,  and  in  the  Black 
Hills,  the  Rocky  Mountains  and  the  ranges  of  California  they  are 
often  met. 

VARIOUS  MINOR  SCHISTS. 

Under  this  collective  term  are  assembled  a  series  of  minor  rocks, 
no  one  of  which  compares  in  importance  with  the  schists  already 
mentioned,  but  which  may  be  met  as  subordinate  members  of 
metamorphic  districts.  There  are  also  others  in  considerably 
variety  which  are  esteemed  too  unimportant  for  an  elementary 
book. 

SiO2.         A12O3.      Fe2O3.       FeO.      '  CaO.        MgO.       K2O.      Na2O.     H2O. 
Chlorite  schist. 

1.  49.18         15.09         12.90  10-59         5-22         1.51         3.64         1.87 

2.  47.10  2.14        44.33  0.36        0.13  5.19 
Talc  schist. 

3.  58.66  9.26          4.42  0.94       22.78  4-09 

4.  50.81  4.53  3.52         4.26  31.55  4-42 
Epidote  schist. 

5.  41.28         18.48          9.44         8.20  7.04         7.48         2.21         352         2.74 
Eclogite. 

6.  48.89  14.46  2.00  7.15  13.76          12.21  0.17  1-75  .40 

Glaucophane  schist. 

7.  47.84         16.88  4.99         5.56         11.15*      7.89         0.46         3.20         1.98 


104  A  HANDBOOK  OF  ROCKS. 

i.  Chlorite  schist,  Klippe,  Sweden,  Cronqvist  for  Tornebohn.  Quoted  by  Roth, 
Gesteinsanalysen,  1884,  p.  VIII.  2.  Chlorite  schist,  Foster  Mine,  Mich.,  C.  F. 
Chandler,  GeoL  of  Mich.  I,  91.  3.  Talc  schist,  Fahlun,  Sweden,  Uhde  quoted  by 
Roth,  Gesteinsanalysen,  1861,  56.  4.  Talc  schist,  Gastein,  Austria,  R.  Richter, 
Idem.  5.  Epidote  schist  from  diabase,  South  Mountain,  Pa.,  C.  H.  Henderson, 
Trans.  Amer.  Inst.  Min.  Eng.,  XII,  82.  6.  Eclogite,  Altenburg,  Austria,  Schuster 
Tscher.  Mitt,  1878,  368.  7.  Glaucophane  schist,  Monte  Diablo,  Calif.,  W.  H.  Mel- 
ville, Bull.  GeoL  Amer.  II,  413. 

Comments  on  the  Analyses.  These  analyses  are  too  variable  to 
admit  of  much  in  the  way  of  comparative  remarks,  for  the  rocks 
are  so  totally  unlike.  No.  I  suggests  an  original  diabase  or  some 
such  rock.  No.  2  is  abnormally  rich  in  iron,  doubtless  in  large 
part  from  magnetite  or  hematite.  The  high  magnesia  in  Nos.  3 
and  4  is  characteristic  and  indicates  their  close  relations  with  ser- 
pentines. No.  5  is  an  altered  diabase.  No.  6  is  of  a  rock  variable 
in  its  mineralogy  and  obscure  in  its  history.  No.  7  is  practically 
a  hornblende-schist  with  glaucophane,  an  amphibole  that  is  high 
in  soda,  instead  of  common  hornblende. 

Mineralogical  Composition,  Varieties.  The  chlorite  schists  are 
marked  by  the  presence  of  this  green  micaceous  mineral  in  large 
amount.  More  or  less  quartz  is  also  generally  present,  and  not  in- 
frequently plagioclase,  talc,  epidote  and  magnetite.  The  schistose 
texture  is  pronounced.  The  chlorite-schists  are  manifestly  altera- 
tion products  from  some  rock,  with  abundant  anhydrous,  iron- 
alumina  silicates.  Hornblende-schists,  presumably  from  basic 
igneous  rocks  are  the  general  source.  Certain  chlorite-schists  are 
often  called  "  green  schists." 

Talc-schists  are  characterized  by  sufficient  talc  to  make  this  min- 
eral prominent  and  in  addition  they  have  quartz  as  the  next  most 
abundant  constituent.  Feldspar  may  at  times  be  noted,  and  some 
micaceous  mineral  is  not  rare.  Care  is  necessary  not  to  confuse 
fine  scales  of  the  last  named  with  talc  itself.  Various  accessory 
minerals  likewise  occur,  and  the  magnesian  carbonates,  dolomite 
and  magnefite  are  often  present.  Obviously  the  talc-schists  have 
resulted  from  the  alteration  of  some  rock  with  one  or  more  anhy- 
drous magnesian  silicates  that  lacked  iron.  Tremolite  and  ensta- 
tite  are  the  most  available,  but  the  original  sources  of  these  are 
often  obscure.  Siliceous  dolomites  or  intrusive  pyroxenites  at 
once  suggest  themselves,  but  the  iron  must  of  necessity  have  been 
low,  so  as  not  to  yield  serpentines. 

Epidote-schists  result  when  the  ferro-magnesian  silicates  and  the 


THE  MINOR   SCHISTS.  105 

plagioclases  are  so  favorably  situated  with  reference  to  each  other 
as  to  establish  mutual  reactions.  They  especially  arise  as  phases 
in  the  metamorphism  of  pyroxenic  or  hornblendic  rocks,  such  as 
diabase,  hornblende-schists  and  the  like.  Eclogite  is  a  rock  scarcely 
known  in  America,  having,  as  yet,  only  been  noted  near  the  Wash- 
ington Mine,  Marquette  District,  Mich.  (Geol.  of  Wis.,  Ill,  649). 
It  is  a  well  recognized  variety,  however,  in  Europe.  It  consists  of 
bright  green  amphiboles  and  pyroxene,  of  garnet  and  of  a  variety 
of  minor  minerals.  In  ordinary  determination  it  would  not  be 
distinguished  from  a  garnetiferous,  actinolite  schist.  Glaucophane 
is  a  blue  soda  amphibole  that  is  rare  in  America,  except  in  the 
coast  range  of  California,  where  it  characterizes  certain  important 
schists.  The  rocks  have  a  pronounced  blue  shade,  and  contain  in 
addition  quartz  and  feldspar.  In  California  they  certainly  are  al- 
tered shales.  Graphite  appears  quite  commonly  as  a  characteristic 
mineral  of  certain  schists,  and  may  justify  the  use  of  the  name 
graphite  schist.  More  or  less  mica,  and  always  quartz  and  feld- 
spar are  associated. 

Distribution. — Chlorite-schist  and  talc-schist  are  not  uncommon 
members  of  our  larger  metamorphic  series,  especially  along  the 
Appalachians,  in  New  England  and  around  Lake  Superior. 
Epidote-schist  is  less  common  in  the  same  relations.  The  occur- 
rence of  eclogite  and  glaucophane-schist  has  already  been  cited. 
Graphite-schist  is  not  infrequent  in  the  metamorphosed  Paleozoic 
strata  of  the  East. 


CHAPTER   XL 

THE  METAMORPHIC  ROCKS,  CONTINUED.     THE   ROCKS   PRODUCED 
BY  REGIONAL  METAMORPHISM.     THE  QUARTZITES  AND 
SLATES.      THE   CRYSTALLINE   LIMESTONES  AND 
DOLOMITES,  OPHICALCITES,  SERPEN- 
TINES   AND    SOAPSTONES. 

THE  QUARTZITES. 

SiO2.      A12O3.     Fe2O3.    FeO.     CaO.      MgO.     K2O.     Na2O.    H2O.     Sp.  Gr. 

1.  97.1         1.39         1.25  0.18        0.13 

2.  96.44          1-74  0.33         0.17  O.22         O.I3         O.I9        0.90 

3.  84.52     12.33  2-12      c-31          tr-         al1       °-34      2.31        2.74 

I.  Quartzite,  Chickies  Station,  Penn.,*  Penn.  Geol.  Surv.  Rep.  M.  p.  91.  2. 
Sandstone  partly  altered  to  Quartzite,  Quarry  Mtn.  Ark.,  R.  N.  Brackett  for  L.  S. 
Grisswold,  Geol.  of  Ark.  1890,  III,  140,  161.  3.  Quartzite,  Pipestone,  Minn.  W.  A. 
Noyes  in  Minn.  Geol.  Surv.  I,  198. 

Comments  on  the  Analyses.  There  is  no  essential  difference  in 
the  analyses  of  quartzites  and  sandstones,  as  the  few  quoted  above 
will  show,  but  doubtless  the  resulting  quartzite  is  somewhat  richer 
in  silica  than  the  original  sandstone.  Comparatively  few  analyses 
of  quartzites  have  been  made  in  America. 

Mineralogical  Composition,  Varieties.  The  quartzites  are  meta- 
morphosed sandstones,  and  differ  from  the  latter  principally  in  their 
greater  hardness,  and  to  a  certain  extent  in  their  fairly  pronounced 
crystalline  character.  These  qualities  are  due  to  the  presence  of 
an  abundant  siliceous  cement  that  is  crystalline  quartz,  and  that  is 
often  deposited  around  the  grains  of  quartz  of  the  original  sand- 
stone, so  as  to  continue  their  physical  and  optical  properties.  The 
original  grains  have,  therefore,  had  the  power  of  controlling  the 


*By  a  mistake  in  copying,  somewhat  justified  by  the  arrangement  in  Report  M.,  an 
analysis  of  a  damourite  slate  was  quoted  on  p.  66,  No.  I,  as  this  quartzite.  The  error 
is  noted  in  the  Errata,  at  the  opening  of  the  book. 

1 06 


THE  QUARTZITES.     THE  SLATES  107 

orientation  of  the  molecules  of  the  new  silica  as  it  crystallized. 
When  the  original  sandstone  has  been  argillaceous  the  resulting 
quartzite  contains  mica  and  especially  muscovite,  and  with  increase 
of  the  mica,  such  quartzites  pass  through  the  intermediate  varieties  of 
quartz-schist  into  mica-schists.  A  very  curious  and  more  or  less  mi- 
caceous variety  is  the  so-called  flexible  sandstone  or  itacolumite, 
whose  grains  have  the  power  of  slight  movement  on  one  another 
from  their  loosely  interlocked  arrangement  so  that  thin  slabs  may 
be  bent  through  a  considerable  arc.  Quartzites  also  result  from 
pebbly  sandstones  and  conglomerates,  and  the  pebbles  of  these 
latter  are  often  flattened  by  the  dynamic  movements  with  which 
the  metamorphism  is  at  times  associated.  There  is  no  sharp  line 
of  demarcation  between  quartzites  and  sandstones,  and  while  the 
extremes  of  soft  sandstones  and  hard  quartzites  are  entirely  differ- 
ent, the  determination  of  intermediate  varieties  is  more  or  less  ar- 
bitrary. 

Alteration.  Quartzites  sometimes  soften  to  sand  on  their  out- 
crops, and  in  the  process,  almost  the  last  vestiges  of  alumina  or 
lime  may  be  removed.  In  this  way  the  sands  in  analysis  No.  I, 
p.  63,  were  formed.  In  general,  however,  they  are  excessively 
resistant  rocks,  and  tend  to  form  prominent  ledges. 

Distribution.  Quartzites  occur  in  almost  all  series  of  metamor- 
phosed sediments,  and  as  these  are  best  developed  in  the  later 
Archean,  (Huronian,  Algonkian)  strata,  they  especially  char- 
acterize them.  In  the  metamorphic  belt  in  New  England  and 
down  to  the  southern  Appalachians,  they  are  frequent,  as  well  as  in 
the  Huronian,  around  Lake  Superior  and  Lake  Huron  and  in  the 
similar  areas  of  the  West. 

THE  SLATES. 


Si02. 

A12O3. 

Fe203. 

FeO. 

CaO. 

MgO. 

K20. 

Na20. 

H20. 

I.  66.45 

13.38 

1.71 

1.41 

2.86. 

6.28 

0.05 

0.90 

4-03 

2.    6600 

24.60 

tr. 

tr. 

3.67 

2.22 

3.00 

3-  65-85 

16.65 

53.1 

0.59 

2-95 

3-74 

I-31 

3.10 

4-  64.57 

17.30 

7.46 

1.16 

2.60 

1.99 

.     . 

4.62 

5.  63.31 

16.16 

3-79 

0.15 

4.44 

7.56 

i-54 

2.65 

6.  60.50 

19.70 

7-83 

1.  12 

2.20 

3.18 

2.20 

3-30 

7-  60.32 

23.10 

7-05 

0.87 

3-83 

049 

4.08 

8.  57.00 

20.  IO 

10.98 

1.23 

3-39 

J-73 

I.30 

4.40 

9.  55.88 

21.85 

9-03 

0.16 

1.49 

3.64 

0.46 

3-39 

10.  54.80 

23-I5 

9-58 

i.  06 

2.16 

3-37 

2.22 

3-90 

io8  A  HANDBOOK  OF  ROCKS. 

I.  Slate  Llanberis,  Wales.  Quoted  by  G.  P.  Merrill,  Stones  for  Building  and  Deco- 
ration, p.  421,  also  MnO,  0.91,  CO2,  1.30.  2.  Slate,  Etchemin  Riv.  N.  B.,  T.  S. 
Hunt,  Phil.  Mag.  (4)  VII,  237,  1854.  3.  Roofing  slate,  Westbury,  Can.,  Idem. 
4.  Roofing  slate,  Lehesten,  Germany.  Frick,  quoted  by  Roth,  Gesteinsanalysen,  1861, 
p.  57.  5.  Damourite  slate,  Hensingerville,  Pa.,  Geol.  of  Penn.  Rep.  M.  91.  Note. — 
/This  analysis  was  misquoted  as  an  Argillaceous  sandstone  on  p.  66,  to  which,  however, 
lit  corresponds  quite  well  in  composition.  6.  Roofing  slate,  Wales,  T.  S.  Hunt  as 
TTnder  No.  2.  7.  Slate,  Lancaster  Co.,  Penn.,  also  FeS2  0.09.  See  under  No.  I, 
8,  Roofing  slate,  Angers,  France,  T.  S.  Hunt,  as  under  No.  2.  9.  Blue-black  car- 
bonaceous slate,  Peach  Bottom  slate,  York  Co.,  Penn.,  also  MnO.,  0.586,  CoO.  tr. 
C.  1.974,  FeS2,  0.51,  SO3,  0.022.  See  under  No.  I,  10,  Roofing  slate,  Kingsey,  Quebec, 
T.  S.  Hunt,  as  under  No.  2. 

Comments  on  the  Analyses.  These  analyses  are  especially  signifi- 
cant when  compared  with  those  of  the  shales  and  clays,  p.  66, 
and  with  those  of  the  mica-schists,  p.  99,  with  which  latter  they 
are  closely  parallel.  Two  features  at  once  impress  the  observer, 
the  excess  of  magnesia  over  lime,  and  the  excess  of  potash  over 
soda.  The  former  stamps  their  origin  as  from  sediments  rather  than 
from  igneous  rocks  of  these  percentages  in  silica,  because  this  rela- 
tive excess  of  magnesia  as  noted  under  the  mica-schists  is  rather 
characteristic  of  sediments. 

Mineralogical  Composition.  Varieties.  As  the  sandstones  during 
metamorphism  pass  into  quartzites,  so  the  shales  and  clays  become 
slates,  when  not  so  thoroughly  recrystallized  as  to  yield  mica- 
schists  or  phyllites.  The  more  sandy  shales  afford  varieties  that 
break  irregularly  and  that  lack  homogeneity,  but  tough  and  even 
slates  result  from  homogeneous  clays  and  are  among  the  most 
remarkable  of  rocks.  The  distinctive  feature  of  slates  as  against 
shales  is  the  possession  of  a  new  cleavage  that  may  lie  at  any 
angle  with  the  original  bedding  of  the  rock,  and  that  has  no  defi- 
nite relation  to  it.  The  cleavage  has  been  developed  by  dynamic 
strains  that  have,  beyond  question,  involved  a  shearing  stress  and, 
some  differential  movement  among  the  layers,  though  it  may  have 
been  microscopic.  As  a  matter  of  observation  the  component 
grains  of  slates  have  become  flattened  and  lie  parallel  with  the  new 
cleavage,  and  any  mica  flakes  or  hornblende  needles  that  may  be 
present  lie  along  it. 

Various  explanations  have  been  advanced  for  slaty  cleavage,  and 
its  artificial  production  in  different  substances  has  occupied  several 
investigators.  Based  principally  upon  experiments  performed  by 
Professor  John  Tyndall,  over  forty  years  ago,  it  has  been  usually 
referred  to  a  compressive  force  at  right  angles  to  its  plane.  Tyndall 


THE  SLATES.  109 

subjected  blocks  of  wax  to  pressure,  using  wet  glass  plates  as  his 
buttress  of  resistance.  The  blocks  were  of  course  greatly  reduced 
in  thickness  and  were  force  to  spread  or  bulge  laterally.  Shortly 
afterward  H.  C.  Sorby,  partly  on  the  basis  of  the  flattening  of  the 
component  grains,  and  the  alignment  of  mica  scales,  explained  the 
cleavage  as  due  to  planes  of  weakness  caused  by  this  new  arrange- 
ment. Recently,  G.  F.  Becker  of  the  U.  S.  Geological  Survey  has 
repeated  the  experiments  of  Tyndall  with  modifications.  So  long 
as  the  resisting  glass  plates  were  wet  with  water  the  slaty  cleavage 
was  developed,  but  when  they  were  smeared  with  a  heavy  lubrica- 
ting oil,  although  there  was  lateral  expansion  during  compression, 
no  bulging  took  place  and  no  cleavage  was  developed.  Manifestly 
therefore  the  frictional  drag  of  the  plates  enters  into  the  problem, 
and  although  the  resolution  of  the  forces  involved  is  somewhat 
complex,  a  shearing  stress  results  that  is  a  strong  factor  in  pro- 
ducing the  cleavage.*  In  the  case  of  the  large  beds  or  strata 
which  are  metamorphosed  into  slate  in  Nature,  the  case  is  even 
less  simple,  and  the  contrasts  in  rigidity,  between  the  beds  that 
yield  slates  and  their  enclosing  strata,  are  less  pronounced  than  in 
the  experiment,  but  there  is  little  doubt  that  the  compression  and 
lateral  flow  with  their  attendant  strains,  occasion  the  cleavage,  and 
that  the  flattening  of  the  grains  and  alignment  of  the  scaly  min- 
erals are  attendant  phenomena.  All  slates  have  cross-cleavages, 
also,  more  or  less  well  developed,  and  one  of  these  may  even  be  per- 
fect enough  to  cause  the  slate  to  break  into  small  prisms  available 
for  slate  pencils,  for  which  in  earlier  years  they  were  employed. 
All  slate-quarries  also  show  curly  slates,  where  quartz-veins  or 
sandy  and  harder  streaks  in  the  original  sediment  have  caused  im- 
perfections in  the  cleavage.  It  has  also  been  noted  that  in  some 
quarries  the  available  plates  appear  to  become  thicker  in  depth,  as 
if  the  surface  weathering  had  been  a  factor  in  developing  the 
cleavages.  Though  commonly  drab  to  black,  they  may  be  red, 
green  or  purple. 

Slates  pass  by  all  intermediate  gradations  into  phyllites  and 
mica-schists.  The  word  slate  is  also  loosely  used  for  shales,  that 
have  never  had  any  secondary  cleavage  induced  in  them,  and  this 
is  especially  true  of  the  black,  bituminous  shales  that  occur  with 
coal  seams,  but  in  strict,  geological  use,  the  new  cleavage  and 
metamorphism  should  be  an  essential  of  a  true  slate. 

*G.  F.  Becker,  Finite  homogeneous  Strain,  Flow  and  Rupture  in  Rocks,  Bull.  GeoL 
Soc.  Amer.  IV,  82,  1893. 


i  io  A  HANDBOOK  OF  ROCKS. 

Alteration.  Slates  are  exceedingly  resistant  as  is  shown  by 
their  use  in  thin  slabs  for  roofs,  and  they  often  constitute  prominent 
ledges  or  even  peaks.  They  soften  down  to  a  clay  in  the  last  stages 
of  alteration,  but  always  on  the  outcrop  are  more  tender  than  in 
depth,  so  that  much  dead  work  is  unavoidable  in  opening  quarries. 

Distribution.  Our  most  prominent  slates  are  Cambrian  or 
Ordovician  in  age.  Along  the  Green  Mountains  and  especially 
in  northern  Vermont  they  are  strongly  developed.  Again  in 
eastern  Pennsylvania,  in  Virginia  and  in  Georgia  they  are  met  in 
great  areas.  On  the  south  shore  of  Lake  Superior  merchantable 
grades  have  been  somewhat  developed.  Along  the  western  slopes 
of  the  Sierra  Nevada  Mountains  they  are  a  very  important  rock. 

THE  CRYSTALLINE  LIMESTONES  AND  DOLOMITES. 

Loss. 

Fe.2Or  or 

CaCO3.          MgCO3.          SiO2.          A12O3.          FeO'.          Insol.          H.2O. 

1.  99.51  0.29  0.20 

2.  99.24  0.28  * , ' 

3.  98.43  0.30  0.31  0.38  0.15 

4.  98.21          2.35  0.15  0.35 

5.  98.00  0.57          1.63 

6.  96.82  1.89  O.IO  2.12 

7-       9242  6.47  0.35  0.95 

8.  70.1  25.40  2.40 

9.  54.62  45.04  o.io  0.7 

io.        54.25  44.45  0.60 

I.  Statuary  Marble,  Brandon,  Vt.  Quoted  by  G.  P.  Merrill,  Stones  for  Building 
and  Decoration,  417.  2.  Marble,  Carrara,  Italy,  Idem.  3.  Marble,  Knoxville, 
Tenn.,  Idem,  also  S,  0.014,  Organic  Matter  0.068.  4.  Coarse-grained  black  and 
white  mottled  Marble,  Pickens  Co.,  Ga.,  locally  called  Creole;  Geol.  Surv.  Ga.,  Bul- 
letin I,  87.  5.  White  Marble,  Rutland,  Vt.,  see  under  No.  I.  6.  Coarsely  crystal- 
line white  Marble,  Cherokee  Quarry,  Pickens  Co.,  Ga.,  see  under  No.  4.  7.  White 
Crystalline  Limestone,  Franklin  Furnace,  N.  J.,  Geo.  C.  Stone,  unpublished.  8. 
Crystalline  Magnesian  Limestone,  Tuckahoe7  N.  Y.,  H.  L.  Bowker  for  Lime  Co.  9. 
Crystalline  Dolomite,  so-called  "  Snowflake  Marble,"  XVI.  Ann.  Rep.  Div.  U.  S. 
Geol.  Survey,  Article  "Stone,"  Reprint  p.  41.  io.  Crystalline  Dolomite,  white 
Marble,  Inyo  Co.,  Calif.,  Ann.  Rep.  Calif.  State  Mineralogist,  218. 

Comments  on  the  Analyses.  The  analyses  do  not  differ  essen- 
tially from  those  of  unaltered  limestones  except  so  far  as  the  ones 
in  the  table  are  purer  carbonates  of  lime  and  magnesia.  The  avail- 
able analyses  are  of  merchantable  marbles,  and  in  the  nature  of 
the  case  these  are  derived  from  very  pure  sedimentary  limestones. 
They  are  interesting  as  illustrating  the  series  from  almost  chemi- 
cally pure  carbonate  of  lime  to  one  in  which  the  carbonate  of  mag- 
nesia reaches  the  values  of  typical  dolomite.  Comparison 


CRYSTALLINE  LIMESTONES  AND  DOLOMITES,  in 

with  the  analyses  of  limestones  earlier  given,  on  p.  70,  is  recom- 
mended. It  will  be  seen  that  in  this  case  there  is  apparently  no 
change  in  gross  composition  from  metamorphism,  but  of  course 
the  relations  of  the  silica  and  the  bases  are  different.  In  the  sedi- 
mentary limestones  the  silica  is  largely  in  the  form  of  quartz  and 
in  combination  with  alumina  forming  hydrated  silicates,  such  as 
kaolin.  In  the  crystalline  limestones  it  is  largely  in  silicates  of 
lime,  magnesia  and  alumina,  such  as  tremolite,  pyroxene,  phlogo- 
pite,  etc.,  minerals  whose  formation  has  been  one  of  the  results  of 
metamorphism.  The  percentages  in  the  insoluble  column  do  not 
therefore  indicate  pure  silica.  There  may  be  even  microscopic 
barite  crystals  present. 

Mineralogical  Composition.  Varieties.  The  crystalline  limestones 
and  dolomites  are  metamorphosed  forms  of  the  sedimentary  varie- 
ties earlier  described.  The  change  involved  is,  as  the  name  im- 
plies, one  of  crystallization.  Fossils,  and  to  a  large  degree  bedding 
planes,  are  destroyed  and  a  more  massive  aggregate  of  calcite  or 
dolomite  crystals  results.  Such  carbonaceous  material  as  was  origi- 
nally present  usually  affords  streaks  of  graphite  which  occasion 
dark  veinings.  They  bring  out  the  brecciation  or  flow-lines  in- 
duced by  the  pressure  from  the  mountain- making  upheavals 
usually  attendant  on  the  metamorphism.  Other  bituminous  or 
ferruginous  matter  may  yield  pronounced  colors  of  many  hues. 

If  the  original  limestone  has  been  an  impure  variety  and  has 
contained  silica,  alumina  and  iron  oxides,  as  illustrated  by  the 
analyses  on  p.  70,  these  components  have  furnished  the  neces- 
sary materials  for  the  various  silicates  that  the  metamorphism 
has  caused  to  form.  Tremolite  is  a  common  result,  light-colored 
pyroxenes  are  not  infrequent,  and  phlogopite  and  other  micaceous 
minerals  are  the  most  abundant  of  all.  Large  quarries  always 
show  borders  or  streaks  that  are  characterized  by  these  minerals, 
and  where  the  original  limestone  passed  into  shales  or  sandstones 
at  its  upper  and  under  surface,  these  micaceous  varieties  are 
almost  always  met.  For  ornamental  purposes  the  included 
silicates,  being  except  in  the  case  of  micas,  of  greater  hardness 
than  the  calcite,  serve  to  mar  the  stone. 

Crystalline  limestones  form  more  or  less  extensive  strata  in  the 
midst  of  other  metamorphic  rocks.  Slates,  phyllites,  mica-schists 
and  quartzites  are  their  most  common  associates.  The  dolomites 
may  have  formed  in  many  cases  from  pure  calcareous  limestones  by 
the  infiltration  of  magnesian  solutions,  and  by  an  exchange  of  a  por- 


112 


A   HANDBOOK  OF  ROCKS. 


tion  of  the  magnesia  for  a  portion  of  the  lime,  as  earlier  referred  to 
on  page  71,  but  so  many  unaltered  limestones  are  high  in  magnesia, 
that  the  change  is  not  a  necessary  attendant  of  metamorphism. 

Alteration.  Crystalline  limestones  are  soluble  rocks  and  weather 
with  comparative  facility.  Where  they  occur  in  metamorphic 
belts  they  are  invariably  in  the  valleys,  and  are  potent  factors  in 
determining  the  direction  of  the  drainage  lines.  Where  exposed 
for  long  periods  they  afford  a  coarse,  crumbling  sand  or  gravel, 
that  is  much  used  for  roads  in  the  borders  of  the  Adirondacks  and 
in  western  New  England.  The  final  stage  is  a  mantle  of  residual 
clay  from  which  the  calcareous  material  has  been  largely  leached. 

Occurrence.  The  crystalline  limestones  are  frequent  in  our  me- 
tamorphic districts.  In  the  Appalachian  belt  they  are  of  great 
areal  and  economic  importance,  and  are  largely  quarried  in  Ver- 
mont, Massachusetts,  New  York,  Pennsylvania  and  Georgia.  In 
western  Colorado  they  are  strongly  developed,  and  in  the  Sierras 
of  California  the  same  is  true,  Inyo  County  being  a  rather  large 
producer  of  marble.  The  foreign  mountainous  and  metamorphic 
districts  exhibit  enormous  exposures.  The  great  series  of  ranges 
that  begin  in  the  Pyrenees  and  extend  through  the  Alps  and  the 
Carpathians  to  the  Himalayas,  have  many  famous  quarries  and 
ledges.  The  region  of  the  "Dolomites"  in  the  Tyrolese  Alps  is 
a  district  of  especial  richness.  The  Carrara  marble  of  the  Ap- 
penines,  the  Pentelic  of  Greece  and  the  colored  varieties  from 
Northern  Africa,  indicate  their  presence  in  those  regions. 

THE  OPHICALCITES,  SERPENTINES  AND  SOAPSTONES. 


Ophicalc. 

CaC03.     MgC03 

.     FeC03.       Si02. 

MgO. 

H20. 

FeO. 

M203. 

I. 

57-37 

9.64 

0.74 

13.18 

10.29 

4.06 

3-57 

0.85 

2. 

23-85 

22.28 

1.97 

22.42 

18.74 

6-43 

4-3° 

3- 

7.65 

10.98 

1.78 

36.53 

28.08 

8.63 

6.49 

Serp. 

SiO2. 

MgO.       H2O.       Al 

A-       Cr20s 

.Fe203. 

FeO. 

NiO. 

CaO. 

4- 

44.14 

42.97 

12.89 

5- 

43-87 

38.62 

9-55 

0.31 

7.17 

0.27 

0.02 

6. 

42.52 

42.16 

14.22 

1.96 

7- 

41-54 

40.42 

14.17 

2.48 

J-37 

0.04 

8. 

40.67 

32.61 

12.77 

5-13 

8.12 

9- 

40.06 

39.02 

I2.IO 

1.37            O.2O 

3-43 

0.71 

10. 

36.95 

33-07 

10.40 

16.50 

ii. 

34-84 

3°-74 

17-39 

0.42        0.68 

6.08 

1.85 

tr. 

7-02 

Soapst. 

MnO. 

12. 

64.44 

33-19 

0-34 

0.48 

i-39 

0.23 

IS- 

62.10 

32.40 

2.05 

1.30 

2.15 

14. 

62.00 

33-i 

4-9 

OPHICALCITES,  SERPENTINES,  SOAPSTONES.    113 

i.  Ophicalcite,  Oxford,  Quebec,  T.  S.  Hunt,  Amer.  Jour.  6W.,  March,  1858,  220. 
The  analysis  as  cited  is  assembled  from  several  partial  analyses.  2.  Ophicalcite, 
Brompton  Lake,  Quebec,  Idem,  p.  221.  Original  results  recast  as  in  No.  I.  3.  Ophi- 
calcite, Brompton  Lake,  Quebec,  Idem,  p.  222.  Recast  as  before.  4.  Theoretical 
serpentine,  H4Mg3SiO9.  5.  Massive  serpentine,  Webster,  N.  C,  F.  A.  Genth, 
Amer.  Jour.  Sc.,  II,  xxxiii,  201.  6.  Massive  serpentine,  Montville,  N.  J.,  E.  A. 
Manice,  Dana's  Mineralogy,  1877,  467.  7.  Serpentine,  a  metamorphosed  sandstone, 
New  Idria,  Calif.,  W.  H.  Melville  for  G.  H.  Becker,  in  Monograph,  XIII.,  U.  S.  Geol. 
Surv.,  no.  8.  Serpentine,  decomposed  peridotite,  Syracuse,  N.  Y.,  T.  S.  Hunt, 
Amer.  Jour.  Sci.,  Sept.,  1858,  237.  9.  Serpentine,  Dublin,  Harford  Co.,  Md.  Quoted 
by  G.  P.  Merrill,  Stones  for  Building  and  Decoration,  414.  10.  Serpentine  from 
peridotite,  Presq' Isle,  Mich.,  J.  D.Whitney,  Amer.  Jour.  Sci.,  II.,  xxviii.,  1 8,  also 
Na2O,  0.97.  n.  Serpentine,  from  peridotite,  Monte  Diablo,  Calif.,  W.  H.Melville, 
Bull.  Geol.  Soc.  Amer.,  II.,  408,  also  Na2O,  0.42,  K2O.  0.07.  12.  Soapstone,  Web- 
ster, Jackson  Co.,  N.  C.,  F.  A.  Genth,  Minerals  of  North  Carolina,  p.  61.  13.  Talc, 
Gouverneur,  N.  Y.,  Analysis  quoted  by  C.  H.  Smyth,  Jr.,  Sch.  of  Mines  Quarterly, 
July,  1896,  p.  340.  14.  Theoretical  talc,  6MgO,  5SiO2,  2H2O. 

Comments  on  the  Analyses.  The  ophicalcites  mark  a  passage 
from  the  dolomites  to  the  serpentines.  They  are  practically  crys- 
talline magnesian  limestones  or  dolomites,  that  are  mottled  with  in- 
clusions of  serpentine  in  varying  amounts.  The  analyses  begin  with 
one  that  is  over  half  calcite  and  over  two-thirds  calcite  and  dolo- 
mite. The  ratios  of  the  remaining  oxides  are  just  about  those  re- 
quired by  serpentine.  In  the  second  the  amount  of  serpentine  has 
much  increased  and  in  the  third  the  carbonates  have  notably  re- 
treated. Under  the  serpentines,  as  compared  with  the  theoretical 
mineral,  No.  4,  the  succeeding  analyses  are  all  notably  rich  in  iron. 
Except  in  the  cases  of  Nos.  10  and  n,  they  are  remarkably  uni- 
form considering  their  diverse  origin.  In  No.  10  the  SiO2  drops, 
probably  from  the  presence  of  magnetite  while  in  the  last  the  py- 
roxene of  the  original  peridotite  has  contributed  considerable  lime. 
In  all  these  rocks  A12O3  is  notably  low.  It  is  most  abundant  in 
No.  8,  a  serpentine  that  is  derived  from  a  rock  with  much  augite. 
Chromium  is  a  rather  characteristic  element  in  serpentines  that 
result  from  basic  igneous  rocks,  and  nickel  can  be  very  generally 
detected  on  analysis.  Lime  practically  fails  except  in  No.  II.  It 
should  be  appreciated  that  as  a  mineral,  serpentine  is  a  unisilicate, 
whereas  talc  is  a  bisilicate,  and  this  explains  the  much  larger  per- 
centage of  silica  in  the  latter.  The  soapstones  are  fairly  pure, 
aggregates  of  talc,  as  a  comparison  of  Nos.  12  and  13  with  No.  14 
will  indicate. 

Mineralogical  Composition.  Varieties.  The  ophicalcites  are  mottled 
rocks  consisting  of  irregular  or  rounded  masses  of  green  serpen- 


ii4  A  HANDBOOK  OF  ROCKS. 

tine  embedded  in  white  calcite  and  dolomite.  The  proportions  of 
the  constituent  minerals  are  variable.  The  serpentine  may  be  in 
small  nodules  a  fraction  of  an  inch  in  diameter  or  in  large  stringers 
and  masses  several  feet  across.  This  irregularity  renders  it  dif- 
ficult in  quarrying  to  preserve  a  uniform  grade.  The  stone  is 
mottled  green  and  white,  and  when  uniform  is  a  very  beautiful  one. 
The  serpentine  varies  from  dark  green  or  almost  black,  to  light 
clear  shades,  and  has  been  derived  in  a  number  of  cases,  as  has 
been  shown  by  G.  P.  Merrill,*  from  original  pyroxene  crystals. 

The  ophicalcites  are  therefore  in  many  cases  alteration  products 
from  a  crystalline  limestone,  that  has  been  surcharged  with  pyrox- 
enes, and  this  itself  may  probably  be  referred  in  most  cases  to  an 
original  siliceous,  magnesian  sediment,  recrystallized  by  regional 
metamorphism. 

Ophicalcites  are  also  called  ophiolites,  serpentinous  marbles 
and  verd  antique.  The  syllable  "  ophi,"  in  all  these  words  is 
derived  from  the  Greek  for  serpent  and  ophicalcite  means  there- 
fore a  serpentinous  limestone. 

The  serpentines  are  green  or  red  aggregates  of  scales,  fibres  or 
massive  individuals  of  the  mineral  serpentine.  They  display  con- 
siderable variety  of  texture  according  to  the  characters  of  these 
components.  Other  minerals  are  not  especially  prominent.  Grains 
of  chromite  or  magnetite  may  be  detected  and  garnets  of  the 
variety  pyrope  are  sometimes  well  developed.  Veinlets  of  calcite  or 
of  magnesian  carbonates  ramify  through  the  rock  in  many  expos- 
ures. Remains  of  the  original  olivine,  pyroxene,  or  hornblende 
from  which  the  serpentine  has  been  derived  may  often  be  detected 
and  biotite  or  some  hydrated  magnesian  mica  is  not  infrequent. 
The  varieties  of  the  mineral  serpentine  are  numerous,  but  many  of 
them  are  too  rare  to  be  serious  rock-makers.  Almost  all  serpen- 
tines have  been  formed  by  the  alteration  of  basic  igneous  rocks, 
among  which  the  pyroxenites  and  peridotites  are  the  chief  con- 
tributors. Hornblende  schists  also  yield  them  and  G.  F.  Becker 
has  recorded  the  remarkable  case  of  sandstones  that  pass  into 
them  in  the  Coast  Ranges  of  California. 

Soapstones,  called  also  steatites,  are  chiefly  talc  as  the  analyses 
show.  Quartz  veinlets  often  run  through  the  rock  and  scattered 
grains  of  quartz  are  not  infrequent.  Magnesian  carbonates 

*G.  P.  Merrill,  Amer.  Jour.  Set.,  March,  1889;  Proc.  U.  S.  Nat  I.  Museum,  XII, 
595,  1890. 


OPHICALCITES,  SERPENTINES,  SOAPSTONES.     115 

are  likewise  evident  in  many  exposures.  In  the  case  of  the 
Gouverneur  beds  of  talc  (see  Anal.  13),  C.  H.  Smyth  has  shown 
that  the  original  minerals  have  been  tremolite  and  enstatite, 
and  that  the  beds  occur  in  crystalline  limestone,  but  from  what  the 
tremolite  and  enstatite  have  been  derived  is  a  hard  problem.  Two 
reasonable  sources  suggest  themselves,  either  a  siliceous  dolomite, 
or  a  non-ferruginous,  basic  intrusive.  The  soapstones  are  not  par- 
ticularly abundant  rocks  but  are  of  economic  value  where  met. 
They  are  close  relatives  of  the  talc  schists  earlier  cited. 

Alteration.  The  serpentinous  rocks  themselves  are  thoroughly 
altered  derivatives  from  fresher  anhydrous  ones  and  in  their  further 
decomposition  simply  soften  to  incoherent  dirt  and  clay.  The 
more  resistant,  included  minerals  are  thus  set  free,  and  as  in  the 
case  of  platinum  and  garnets  they  may  be  concentrated  in  gravel. 

Distribution.  Ophicalcites  are  most  abundant  in  Quebec,  the  north- 
ern Green  Mountains  and  the  foothills  of  the  Adirondacks.  The 
serpentines  are  especially  notable  on  Staten  Island,  in  southeastern 
Pennsylvania  and  the  neighboring  parts  of  Maryland,  where  the 
gabbros,  as  stated  on  p.  52,  and  their  related  rocks  are  abundant. 
They  share  in  an  important  belt  of  these  basic  intrusives  in  North 
Carolina  and  Georgia.  In  the  basic  igneous  rocks  around  Lake 
Superior,  they  are  occasionally  met  as  alteration  products.  In  the 
Coast  ranges  the  serpentines  are  of  very  great  importance,  and  in 
part  are  altered  sediments.  They  are  likewise  common  abroad, 
and  in  a  minor  capacity  appear  in  many  metamorphic  districts. 
Soapstone  is  much  less  common,  but  is  met  in  this  country  as  a 
minor  member  in  much  the  same  regions  as  the  serpentines  and 
crystalline  dolomites. 


. 


CHAPTER  XII. 

THE  METAMORPHIC  ROCKS,  CONCLUDED.     THE  ROCKS   PRODUCED 

BY  ATMOSPHERIC  WEATHERING.     THE  DETERMINATION 

OF  THE  METAMORPHIC  ROCKS.' 

Introduction.  It  is  a  matter  of  common  observation  that  out- 
crops of  rock  and  loose  boulders  are  always  more  or  less  decom- 
posed and  broken  down  or  "  weathered  "  for  a  greater  or  less  dis- 
tance below  their  surfaces.  This  may  not  be  serious  enough  to 
prevent  the  accurate  recognition  of  the  rock,  and  usually  within 
the  area  once  covered  by  the  great  ice  sheet  of  the  Glacial  Period  it 
is  not,  because  the  moving  ice  has  ploughed  away  all  loose  and  de- 
composed materials,  but  south  of  the  terminal  moraine,  and  above 
all  in  the  tropics,  the  decomposition  is  excessive  and  may  produce 
to  a  depth  of  a  hundred  feet  or  more  a  mass  of  alteration  pro- 
ducts that  give  of  themselves  slight,  if  any,  clue  to  their  originals. 
This  is  a  common  experience  in  the  Southern  States,  where,  as 
well  as  in  Central  and  South  America,  the  indefinite  character  of 
the  surface  rock  throws  great  difficulties  in  the  way  of  accurate 
geological  mapping.  So  difficult  at  times  is  the  determination  of 
the  country  rock  that,  for  example,  during  field  work  in  Brazil,  O. 
A.  Derby  has  felt  compelled  to  resort  to  the  panning  out  of  the 
surface  materials  with  a  gold-seeker's  pan  in  order,  by  concentra- 
ting the  heavy  but  small  and  undecomposed  accessory  minerals, 
such  as  zircon,  titanite,  monazite,  xenotime,  apatite  and  others,  to 
get  some  clue  from  their  characteristic  associations  to  the  original 
rock.  Many  travelers  have  noted  the  brilliant  colors  of  the  soils 
of  latitudes  toward  the  equator  and  the  comparatively  somber 
tones  of  those  toward  the  poles. 

These  products  of  weathering  are  so  widespread,  therefore,  and 
so  individual  that  a  few  pages  have  been  reserved  for  their  particu- 
lar mention.  Special  names  for  them  have  been  suggested  at  various 
times.  The  oldest  one  and  the  one  most  current  is  laterite.  The 

116 


ROCKS  FROM  A  TMO  SPHERIC  WE  A  THERING.     1 1 7 

word  means  brick  earth  and  was  originally  applied  to  the  red  or 
brown  ironstained  surface  soils  occurring  in  the  tropical  lands,  and 
derived  by  direct  decomposition  from  the  country  rock  in  place. 
It  has  been  applied  in  later  years,  however,  to  all  sorts  of  these  sur- 
face soils  from  whatever  rocks  derived,  and  whether  colored  red  or 
not.  G.  F.  Becker,  of  the  U.  S.  Geological  Survey,  has  recently 
(1895)  proposed  saprolite,*  a  word  meaning  literally  rotten  rock, 
as  "  a  general  name  for  thoroughly  decomposed,  earthy,  but  un- 
transported  rock."  This  is  practically  the  modern  use  of  laterite, 
although  it  is  broader  than  the  latter's  original  application.  The 
U.  S  Geological  Survey  in  the  invaluable  series  of  atlas  sheets  now 
being  issued  employs  the  term  "  surficial,"  i.  e.,  surface  rocks,  as  a 
general  designation  for  these  untransported  products  of  decom- 
position. We  also  often  speak  of  residual  clay  as  was  done  on  pp. 
66  and  67  for  the  less  soluble  aluminous  residues  left  behind  in 
the  removal  of  the  more  soluble  portions  of  limestones. 

The  general  scope  and  application  of  these  names  having  been 
set  forth,  a  brief  consideration  will  be  given  to  the  mineralogical 
processes  of  change  that  have  produced  them  from  several  of  the 
commoner  groups  of  rock. 

The  chief  causes  of  this  superficial  breaking  down  or  "  degenera- 
tion," as  it  has  been  aptly  called  by  G.  P.  Merrill,f  are  the  chemi- 
cal action  of  rain  and  ground-waters,  especially  when  charged  with 
carbonic  acid  or  other  dissolved  matter  ;  organic  life,  both  vege- 
table and  animal,  operating  through  the  agency  of  the  organic  acids 
produced  by  their  living  processes  or  by  their  decomposing  re- 
mains ;  and  the  mechanical  disintegration  produced  by  changes 
of  temperature,  by  the  freezing  of  water  and  by  swelling  from  hy- 
dration  or  from  some  of  the  chemical  or  mineralogical  changes 
among  those  referred  to  above.  Although  having  no  connection 
with  these  atmospheric  processes,  yet  hot  springs  and  allied  exhala- 
tions from  dying  volcanic  activity  bring  about  closely  similar  re- 
sults and  are  able  to  change  great  sheets  of  volcanic  rock  to  bril- 
liantly variegated  masses  of  clay  and  kaolin.  At  the  Falls  of  the 
Yellowstone  River,  in  the  National  Park,  these  are  wonderfully  and 
impressively  displayed,  more  than  a  thousand  feet  of  rhyolite 
having  been  changed  practically  to  kaolin. 

Under  the  action  of  the  chemical  agents  the  more  easily  soluble 

*Goldfields  of  the  Southern  Appalachians,  p.  43,  XVI.  Ann.  Rep.  Dir.  U.  S.  Geol. 
Survey. 

\  Bulletin  of  the  Geological  Society  of  America,  VII.,  378. 


1 1 8  A  HANDBOOK  OF  ROCKS. 

elements  are  removed  or  are  put  in  such  relations  to  one  another 
as  to  facilitate  their  rearrangement  in  new  and  secondary  com- 
binations. In  the  rocks  composed  of  silicates  the  most  vulnerable 
oxides  are  lime,  magnesia,  potash  and  soda.  Iron  oxides  also 
suffer  extensively,  but  the  ferric  form  is  sometimes  very  resistant. 
Silica  yields  more  or  less,  especially  to  the  alkaline  solutions  from 
the  potash  and  soda  referred  to  above.  Alumina,  on  the  whole, 
is  least  readily  attacked  of  all,  and  is  usually  the  one  that  furnishes 
the  best  basis  of  comparison  between  analyses  of  altered  and  un- 
altered materials. 

Among  the  igneous  and  metamorphic  rocks  open  or  porous 
varieties  naturally  suffer  more  than  compact  and  finely  crystalline 
ones.  Rocks  high  in  the  bases  that  are  most  readily  attacked 
chemically,  are  easier  victims  than  those  especially  rich  in  the  re- 
sistant ones.  Basic  rocks,  therefore,  with  their  high  percentages 
of  lime  and  magnesia  and  their  relatively  low  silica,  suffer  espe- 
cially, whereas  granites  and  related  gneisses  are  much  more  stub- 
born subjects,  the  large  amount  of  quartz  in  them  furnishing  a 
very  resistant  component. 

Granites,  syenites,  acid  diorites  and  their  corresponding  porphy- 
ritic  types  alter  especially  through  the  feldspathic  member  present. 
The  constitueut  quartz  is  but  slightly  affected,  and  the  dark  silicates 
are  not  present  in  sufficiently  large  amounts  to  be  very  serious  fac- 
tors. The  resulting  product  is  a  kaolinized  or  clayey  mass  through 
which  are  distributed  quartz  grains,  and  which  is  more  or  less 
stained  by  the  hydrated  oxide  of  iron  that  is  yielded  to  some  ex- 
tent by  the  dark  silicates.  The  characteristic  products  of  the  latter 
are  also  present  in  small  amounts,  but  are  more  extensively  men- 
tioned subsequently.  The  exposed  ledges  furnish  loose  pieces 
that  often  weather  in  concentric  shells  and  simulate  rounded  water- 
worn  boulders.  The  net  result  is  a  large  contribution  of  clay  and 
sand  to  sedimentary  or  eolian  deposits  it  may  be  at  a  great  distance. 

In  the  basic  igneous  or  metamorphic  rocks  the  dark  ferro-mag- 
nesian  and  aluminous  silicates  are  in  excess,  and  in  decomposition 
their  peculiar  products  predominate.  The  distinctively  magnesian 
ones  yield  serpentine,  the  aluminous  change  to  chlorite.  Both 
these  minerals  are  prevailingly  green,  and  dark  green  surficial  rocks 
result.  The  abundance  of  iron  in  them  leads  to  the  formation  of 
very  rusty  outcrops. 

In  the  case  of  limestone,  the  lime  and  magnesia  are  dissolved 


,     DETERMINA  TION  OF  METAMORPHIC  ROCKS.     1 1 9 

away,  while  the  alumina,  silica  and  iron  oxides  remain  behind  in 
the  mantle  of  impure  residual  clay  already  referred  to ;  the  other 
sedimentary  rocks  suffer  especially  from  mechanical  processes, 
although  chemical  changes  are  not  lacking  among  them,  for,  as  re- 
marked on  page  107  regarding  analysis  No.  I,  of  page  63,  during 
the  breaking  up  considerable  leaching  may  result  that  leads  to  the 
production  of  nearly  chemically  pure  quartz  sand. 

The  mechanical  and  associated  chemical  breaking  down  of  rocks 
tends  to  place  them  in  more  favorable  conditions  for  further  chem- 
ical alterations,  and  for  erosion  and  removal. 

All  the  changes  in  the  weathering  of  rocks  have  been  well  de- 
scribed by  M.  E.  Wadsworth  as  "  resulting  from  the  general  dissi- 
pation and  degradation  of  the  potential  energy  of  the  constituents 
of  the  earth's  crust  in  the  universal  passage  of  matter  from  an  ac- 
tive state  towards  a  passive  and  inert  condition."* 


THE  DETERMINATION  OF  THE  METAMORPHIC  ROCKS. 

The  rocks  resulting  from  contact  metamorphism  are  rather  of 
local  interest,  than  of  wide,  areal  distribution.  The  spotted  schists 
and  slates,  and  the  hornstones  are  readily  recognized  by  a  practiced 
observer.  The  crystalline  limestones  even  when  charged  with 
silicates  may  closely  resemble  the  products  of  regional  metamor- 
phism. In  dealing  with  the  latter,  familiarity  with  well  character- 
ized types  is  the  safest  guide.  The  gneisses  are  at  once  apparent 
from  their  laminated  character  and  granitoid  texture.  Transition 
members  between  them  and  the  mica-schists  on  the  one  hand, 
and  the  hornblende-schists  on  the  other,  may  cause  hesitation  as 
to  which  group  they  belong  to.  The  finely  laminated  ones  are 
certainly  members  of  the  schists,  those  with  prevailing  mica  be- 
longing with  the  mica-schists,  those  with  prevailing  hornblende, 
with  the  hornblende-schists.  Again  as  the  fineness  of  the  lamina- 
tion or  foliation  increases,  the  schists  pass  into  the  phyllites  and 
slates,  that  are  easily  recognized.  The  quartzites  likewise  present 
little  difficulty  as  they  are  practically  hard  sandstones.  The  crys- 
talline limestones  and  dolomites  are  only  to  be  distinguished  by 
the  ease  or  difficulty  of  obtaining  effervescence.  The  ophicalcites 
look  like  no  other  rocks,  and  the  serpentines  and  soapstones  are 

*The  Theories  of  Ore  Deposits,  Proc.  Bost.  Soc.  Nat.  Hist.     Vol.  XXIII.,  p.  202. 
1884. 


120  A  HANDBOOK  OF  ROCKS. 

also  at  once  apparent.  The  soapy  feel  of  all  these  magnesian 
rocks  aids  in  their  recognition.  There  are,  of  course,  rare  and 
obscure  metamorphic  rocks  that  cause  trouble,  but  such,  just  as  in 
the  case  of  the  finely  crystalline  igneous  rocks,  are  best  referred 
to  someone  familiar  with  the  use  of  the  microscope. 


GLOSSARY. 


NOTE. — In  the  following  definitions,  when  fuller  explanations  are  to  be  found  in 
preceding  pages,  references  are  given  to  them  and  they  should  be  consulted.  No  at- 
tempt has  been  made  to  unnecessarily  repeat  previous  statements. 


A 

Absarokite,  a  general  name  given  by  Iddings  to  a  group  of  igneous 
rocks  in  the  Absaroka  range,  in  the  eastern  portion  of  the  Yellowstone 
Park.  They  have  porphyritic  texture  with  phenocrysts  of  olivine  and 
augite  in  a  groundmass,  either  glassy  or  containing  leucite,  orthoclase  or 
plagioclase,  one  or  several.  They  range  chemically,  SiO2,  46-52 ;  A12O3, 
9-12  ;  MgO,  8-13  ;  alkalies,  5-6.3  with  potash  in  excess.  The  name  is 
of  greatest  significance  when  taken  in  connection  with  shoshonite  and 
banakite.  Jour,  of  Geol.  III.,  936. 

Ablation,  a  name  applied  to  the  process  whereby  residual  deposits 
are  formed  by  the  washing  away  of  loose  or  soluble  materials. 

Abyssal-rocks,  synonym  of  plutonic  rocks  as  used  in  preceding  pages. 
The  word  has  been  suggested  and  especially  used  by  W.  C.  Brogger. 
^  Accessory  components  or  minerals  in  rocks  are  those  of  minor  im- 
portance or  of  rare  occurrence,  whose  presence  is  not  called  for  by  the 
definition  of  the  species. 

Acidic,  a  descriptive  term  applied  to  those  igneous  rocks  that  contain 
more  than  65  %  SiO2,  as  contrasted  with  the  medium  of  65  %-$$  %  and 
the  basic  at  less  than  55  %  •  still  the  limits  are  somewhat  elastic. 

Acmite-trachyte,  a  trachyte  whose  pyroxene  is  acmite  or  segirine 
and  whose  feldspar  is  anorthoclase.  It  therefore  differs  from  normal 
trachyte  in  its  prevailing  soda  instead  of  potash,  as  is  shown  by  the  ac- 
mite, a  soda-pyroxene,  and  the  anorthoclase,  a  soda-feldspar.  The  ac- 
mite-trachytes  are  intermediate  between  the  true  trachytes  and  the 
phonolites.  They  were  first  described  from  the  Azores  (Miigge,  Neues 
Jahrbuch.  1883,  II.,  189)  and  have  also  been  found  in  the  Crazy  Moun- 
tains, Mont. ;  see  p.  26,  Anals.  4  and  5. 

Adamellite,  a  name  proposed  by  Cathrein  as  a  substitute  for  tonalite, 
on  the  ground  that  tonalite  is  a  hornblende-biotite  granite,  rich  in  plagio- 
clase, rather  than  a  diorite.  The  name  is  derived  from  Monte  Adamello, 
near  Meran,  Tyrol,  the  locality  of  tonalite.  Neues  Jahrb.  1890,  I.,  75. 
Brogger  uses  it  for  acidic  quartz-monzonite.  Eruptions-folge  bei  Pre- 
dazzo,  61. 

121 


122  A   HANDBOOK  OF  ROCKS. 

Adinole,  a  name  for  dense  felsitic  rocks  composed  chiefly  of  an  aggre- 
gate of  excessively  fine  quartz  and  albite  crystals,  such  that  on  analysis 
the  percentage  of  soda  may  reach  10.  Actinolite  and  other  minerals  are 
subordinate.  Adinoles  occur  as  contact  rocks,  associated  with  diabase 
intrusions  and  produced  by  them  from  schists  (compare  spilosite  and 
desmite);  and  as  individual  beds  in  metamorphic  series.  (Compare  por- 
phyroid,  halleflinta.)  The  name  was  first  given  by  Beudant,  but  has 
been  especially  revived  by  Lessen.  Zeits.  d.  d.  Geol.  Ges.,  XIX.,  572, 
1867. 

Aerolite,  a  synonym  of  meteorite. 

£/  Agglomerate,  a  special  name  for  volcanic  breccias  as  distinguished 
from  other  breccias  and  from  conglomerates. 

Akerite,  a  special  name  coined  by  Brogger  for  a  variety  of  syenite  at 
Aker,  Norway,  that  is  a  granitoid  rock  consisting  of  orthoclase,  consider- 
able plagioclase,  biotite,  augite  and  some  quartz.  (W.  C.  Brogger, 
Zeitsch.  f.  Krys.,  1890,  43.) 

Algovite,  a  name  proposed  by  Winkler,  for  a  group  of  rocks,  practi- 
cally diabases,  or  porphyritic  phases  of  the  same,  in  the  Algauer  Alps. 
They  also  embrace  gabbros  according  to  Roth,  and  are  doubtless  various 
textural  varieties  of  an  augite-plagioclase  magma.  Neues  Jahrbuch, 
1895,  641. 

^  Allotriomorphic,  an  adjective  coined  by  Rosenbusch  in  1887  to 
describe  those  minerals  in  an  igneous  rock  which  do  not  possess  their 
own  crystal  faces  or  boundaries  but  which  have  their  outlines  impressed 
on  them  by  their  neighbors.  They  result  when  a  number  of  minerals 
crystallize  at  once  so  as  to  interfere  with  one  another.  They  are  espe- 
cially characteristic  of  granitoid  textures.  The  word  was  unnecessary,  as 
xenomorphic  had  been  earlier  suggested  for  the  same  thing,  but  it  is  in 
more  general  use  than  xenomorphic.  See  also  anhedron. 

Alluvium,  Lyells'  name  for  the  deposit  of  loose  gravel,  sand  and 
mud  that  usually  intervenes  in  every  district  between  the  superficial  cov- 
ering of  vegetable  mould  and  the  subjacent  rock.  The  name  is  derived 
from  the  Latin  word  for  an  inundation  (Elements  of  Geol.,  6th  Ed.,  N. 
Y.,  1859,  p.  79).  Used  by  Naumann  as  a  general  term  for  sediments 
in  water  as  contrasted  with  eolian  rocks.  It  is  generally  used  to-day  for 
"  the  earthy  deposit  made  by  running  streams  or  lakes,  especially  during 
times  of  flood."  (Dana's  Manual,  1895,  P-  81.)  ^-n  a  stratigraphical 
sense  it  was  formerly  employed  for  the  more  recent  water-sorted  sedi- 
ments as  contrasted  with  ' 'diluvium,"  or  the  stratified  and  unstratified  de- 
posits from  the  melting  of  the  continental  glacier  of  the  Glacial  Period. 
This  use,  with  fuller  study  of  the  Glacial  times,  is  practically  obsolete. 

Alnoite,  a  very  rare  rock  with  the  composition  of  a  melilite  basalt, 
that  was  first  discovered  in  dikes  on  the  island  of  Alno,  off  the  coast  of 


GLOSSARY.  123 

Norway.  The  special  uame  was  given  it  by  Rosenbusch  to  emphasize  its 
occurrence  in  dikes  and  its  association  as  a  very  basic  rock,  with  nephe- 
line  syenite.  Alnoite  has  been  discovered  near  Montreal  by  F.  D.  Adams. 
(Amer.  Jour.  Sci.  April,  1892,  p.  269.)  and  at  Manheim  Bridge,  N.  Y., 
by  C.  H.  Smyth,  Jr.  (Amer.  Jour.  Sci.  Aug.,  1893,  104). 

Alsbachite,  a  name  given  by  Chelius  to  a  variety  of  granite-por- 
phyry, forming  dykes  in  Mt.  Melibocus,  and  containing  large  mica 
•crystals  and  rose-red  garnets.  Notizbl.  Ver.  Erdk,  zu  Darmstadt,  1892. 
Heft.  13,  i. 

Alum-shales,  shales  charged  with  alum  which  in  favorable  locali- 
ties may  be  commercially  leached  out  and  crystallized.  The  alum  results 
from  the  decomposition  of  pyrites.  The  sulphuric  acid  thus  produced 
reacts  on  the  alumina  present,  yielding  the  double  sulphate  that  is  alum. 

^  Amphibole,  the  generic  name  for  the  group  of  bisilicate  minerals 
whose  chief  rock-making  member  is  hornblende.  It  is  often  prefixed  to 
those  rocks  which  have  hornblende  as  a  prominent  constituent,  as  amphi- 
bole-andesite,  amphibole-gabbro,  amphibole-granite,  etc. 

Amphibolite,  a  metamorphic  rock  consisting  chiefly  of  hornblende, 
or  of  some  member  of  the  amphibole  group.  It  is  as  a  rule  a  synonym  of 
hornblende-schist,  but  is  preferable  to  the  latter  when  the  schistosity  is 
not  marked.  See  p.  101. 

>^  Amygdaloids  are  cellular  lavas,  whose  cavities,  caused  by  expand- 
ing steam-bubbles,  resemble  an  almond  in  size  and  shape.  Basaltic  rocks 
are  most  prone  to  develop  them.  The  term  is  used  in  the  form  of  the 
adjective,  amygdaloidal,  and  properly  should  be  limited  to  this.  As  a 
noun  it  is  also  employed  for  secondary  fillings  of  the  cavities,  which  are 
usually  calcite,  quartz  or  some  member  of  the  zeolite  group.  Amygda- 

.  loidal  rocks  are  of  chief  interest  in  America,  because  certain  basaltic 
lava  sheets  on  Keweenaw  Point,  Lake  Superior,  have  their  amygdules 
filled  with  native  copper  and  are  important  sources  of  the  metal.  Amyg- 
daloidal cavities  are  limited  to  the  upper  and  lower  portions  of  lava 
sheets.  The  name  is  derived  from  the  Greek  word  for  almond. 

/  Analcite-basalt,  a  variety  of  basalt  whose  feldspar  is  more  or  less 
replaced  by  analcite.  The  analcite  is  at  times  in  such  relations  as  to  give 
•reason  for  thinking  it  an  original  mineral  and  not  an  alteration  product 
from  feldspar.  Analcite  basalts  occur  in  the  High  wood  Mountains,  Mont. , 
(see  W.  Lindgren,  loth  Census,  XV.,  727,  Proc.  Calif.  Acad.  Sci.,  Ser. 
II.,  Vol.  III.,  p.  51.  Comptes  Rendus,  Fifth  Internat.  Geol.  Cong.,  364). 
Analcite-diabase  has  been  met  in  California.  (H.  W.  Fairbanks,  Bull. 
Dept.  Geol.  Univ.  of  Calif.,  I.,  173.  See  also  in  this  connection  tes- 
chenite.) 

Anamesite,  an  old  name  suggested  by  von  Leonhard  in  1832,  for 
those  finely  crystalline  basalts,  which  texturally  stand  between  the  dense 


124  A   HANDBOOK  OF  ROCKS. 

typical  basalt,  and  the  coarser  dolerites.     The  name  is  from  the  Greek, 
for  "in  the  middle." 

Andalusite-hornstone,  a  compact  contact  rock  containing  andalu- 
site.  It  is  usually  produced  from  shales  or  slates  by  intrusions  of  granite. 

Andesite,  volcanic  rocks  of  porphyritic  or  felsitic  texture,  whose 
crystallized  minerals  are  plagioclose  and  one  or  more  of  the  following : 
biotitej  hornblende  and  augite.  The  name  was  suggested  by  L.  von  Buch 
in  1836,  for  certain  rocks  from  the  Andes,  resembling  trachytes,  but 
whose  feldspar  was  at  first  thought  to  be  albite,  and  later  oligoclase. 
See  p.  40. 

Anhedron,  a  name  proposed  by  L.  V.  Pirsson  for  the  individual  min- 
eral components  of  an  igneous  rock,  that  lack  crystal  boundaries,  and  that 
cannot  therefore  be  properly  called  crystals  according  to  the  older  and 
most  generally  accepted  conception  of  a  crystal.  Xenomorphic  and  allo- 
triomorphic  are  adjectives  implying  the  same  conception.  The  name 
means  without  planes.  Bulletin  Geol.  Society  of  America,  Vol.  VII.,  p. 
492,  1895. 

Anogene,  a  general  name  for  rocks  that  have  come  up  from  below ;. 
/.  e.,  eruptive  rocks.  See  p.  13. 

Anorthite-rock,  a  name  given  by  R.  D.  Irving  to  a  coarsely  crystal- 
line, granitoid  rock,  from  the  Minnesota  shore  of  Lake  Superior,  that 
consists  almost  entirely  of  anorthite  (Monograph  V.,  U.  S.  Geol.  Survey, 
p.  59).  The  rock  is  a  feldspathic  extreme  of  the  gabbro  group,  practic- 
ally an  anorthosite  formed  of  anorthite. 

Anorthosite,  a  name  applied  by  T.  Sterry  Hunt  (Amer.  Jour.  Sci., 
Nov.,  1869)  to  granitoid  rocks  that  consist  of  little  else  than  labradorite- 
and  that  are  of  great  extent  in  eastern  Canada  and  the  Adirondacks.. 
The  name  is  derived  from  anorthose,  the  French  word  for  plagioclase, 
and  is  not  to  be  confused  with  anorthite,  with  which  it  has  no  necessary" 
connection,  although  anorthosite  is  used  as  a  general  name  for  rocks  com- 
posed of  plagioclase.  Mt.  Marcy  and  the  neighboring  high  peaks  of  the 
Adirondacks  are  formed  of  it.  The  rocks  are  extremes  of  the  gabbro 
group,  into  whose  typical  members  they  shade  by  insensible  gradations. 
See  p.  50. 

Apachite,  a  name  suggested  by  Osann,  from  the  Apache,  or  Davis 
Mountains  of  Western  Texas,  for  a  variety  of  phonolite,  that  varies  from 
typical  phonolites  in  two  particulars.  It  has  almost  as  much  of  amphi- 
boles  and  of  aenigmatite  as  of  pyroxene,  whereas  in  normal  phonolites  the 
former  are  rare.  The  feldspars  of  the  groundmass  are  generally  micro- 
perthitic.  Tscher.  Mitth.,  XV.,  454. 

Aphanite,  an  old  name,  now  practically  obsolete,  for  dense,  dark 
rocks,  whose  components  are  too  small  to  be  distinguished  with  the  eye. 
It  was  chiefly  applied  to  finely  crystalline  diabases.  An  adjective  aphan- 
itic  is  still  more  or  less  current. 


GLOSSARY.  125 

ir.  Aplite  is  now  chiefly  applied  to  the  muscovite-granite  that  occurs  in 
dikes,  and  that  is,  as  a  rule,  finely  crystalline.  Its  original  application 
was  to  granites  poor  or  lacking  in  mica.  See  p.  31.  The  name  is  from 
the  Greek  for  simple. 

Apo,  the  Greek  preposition  for  "  from,"  suggested  by  F.  Bascom  as  a 
prefix  to  the  names  of  various  volcanic  rocks  to  describe  the  devitrified 
or  silicified  varieties,  mostly  of  ancient  date,  that  result  from  them,  and 
that  indicate  their  originals  only  by  the  preservation  of  characteristic 
textures.  Thus  apobsidian,  aporhyolite,  apandesite,  apobasalt,  etc., 
have  been  used.  (See  p.  22.)  Many  rocks  called  by  the  old  indefinite 
name  petrosilex  are  of  this  character.  Journal  of  Geology,  I.,  828, 
Dec.,  1893. 
^  Arenaceous,  an  adjective  applied  to  rocks  that  have  been  derived 

from  sand,  or  that  contain  sand. 
^Argillite,  a  synonym  of  slate. 

^Arkose,  a  special  name  for  a  sandstone  rich  in  feldspar  fragments,  as 
distinguished  from  the  more  common  richly  quartzose  varieties.  See 
p.  64. 

Aschaffite,  a  name  suggested  by  Giimbel  for  a  dike  rock  occurring 
near  Aschaffenburg,  Bavaria.  (Bavaria,  Vol.  IV.,  Heft,  u,  p.  23.)  It 
is  defined  by  Rosenbusch  as  a  dioritic  dike  rock,  containing  quartz, 
plagioclase  and  biotite  as  the  chief  dark  silicate. 

Ashbed  diabase,  a  local  name  used  on  Keweenaw  Point,  Lake  Su- 
perior, for  a  rock  resembling  a  conglomerate,  but  which  is  interpreted  by 
Wadsworth  as  a  very  scoriaceous  amygdaloidal  sheet  into  which  much 
sand  was  washed  in  its  early  history.  See  Monograph  V.,  U.  S.  Geol. 
Surv.,  p.  138. 

Asiderite.  Daubree's  name  for  stony  meteorites  that  lack  metallic 
iron. 

Asperite,  a  collective  name  suggested  by  G.  F.  Becker  for  the  rough 
cellular  lavas  whose  chief  feldspar  is  plagioclase,  but  of  which  it  is  im- 
possible to  speak  more  closely  without  microscopic  determination.  The 
name  is  intended  for  general  field  use  much  as  trachyte  was  employed  in 
former  years,  and  it  is  coined  from  the  Latin  word  for  rough.  See  p.  41. 
Also  Monograph  XIII. ,  U.  S.  Geol.  Surv.,  p.  151. 

Ataxite.    See  under  Taxite. 

Augen,  the  German  word  for  eyes ;  used  as  a  prefix  before  various  rock 
names,  but  more  especially  gneiss,  to  describe  larger  minerals  or  aggre- 
gates of  minerals,  which  are  in  contrast  with  the  rest  of  the  rock.  In  the 
gneisses,  feldspars  commonly  form  the  augen  and  are  lenticular  with  the 
laminations  forking  around  them,  in  a  way  strongly  suggesting  an  eye. 
The  term  is  seldom  used  in  any  other  connection  than  with  gneiss  in 
America. 


126  A   HANDBOOK  OF  ROCKS. 

f     Augite,  the  commonest   rock-making  pyroxene.     The  name  is  used 
as  a  descriptive  prefix  to  many  rocks  that  contain  the  mineral,  as  for  in- 
•  stance  augite-andesite,  augite-diorite,augite-gneiss,  augite-granite,  augite- 
syenite,  etc. 

Augitite,  non-feldspathic,  porphyritic  rocks,  consisting  essentially  of 
a  glassy  groundmass,  with  disseminated  augite  and  magnetite.  Various 
minor  accessories  also  occur.  The  name  was  first  applied  by  Doelter  to 
lavas  from  the  Cape  Verde  Islands.  (Verhandl,  d.  k.  k.  Geol.  Reichs- 
anst.  1882,  143.)  See  above,  pp.  44,  45. 

Aureole,  the  area  surrounding  an  igneous  intrusion  that  is  affected 
by  contact  metamorphism.  Seep.  87. 

Authigenous,  an  adjective  coined  by  Kalkowsky  to  describe  those 
minerals  which  form  in  sediments  after  their  deposition,  as  for  instance 
during  metamorphism.  The  name  emphasizes  in  its  etymology  the  local 
origin  of  the  minerals  as  contrasted  with  that  of  the  other  components, 
they  having  been  brought  from  a  distance. 

Autochthonous,  an  adjective  derived  from  two  Greek  words,  meaning 
indigenous.  It  is  applied  to  those  rocks  that  have  originated  in  situ, 
such  as  rock  salt,  stalagmitic  limestones,  peat,  etc.,  but  it  is  of  rare  use. 
y  Automorphic  is  the  contrasted  term  with  xenomorphic  or  allotrio- 
morphic,  and  is  used  to  describe  those  minerals  in  rocks  which  have  their 
own  crystal  boundaries.  The  later  suggested  word,  idiomorphic,  means 
the  same  thing  and  is  somewhat  more  widely  used. 

Axiolite,  a  term  coined  by  Zirkel  in  his  report  on  Microscopical  Petrog- 
raphy, for  the  U.  S.  Geol.  Survey  along  the  Fortieth  Parallel,  1876,  to 
describe  those  spherulitic  aggregates  that  are  grouped  around  an  axis 
rather  than  around  a  point.  The  application  comes  in  microscopic  work 
rather  than  in  ordinary  determination. 


B 

Banakite,  a  general  name  given  by  Iddings  to  a  group  of  igneous 
rocks  in  the  eastern  portion  of  the  Yellowstone  Park  and  chiefly  in 
dykes.  They  are  porphyritic  and  richly  feldspathic.  The  phenocrysts 
are  labradorite  and  the  groundmass  consists  of  alkali-feldspars.  A  little 
biotite  and  subordinate  augite  may  be  present.  Chemically  they  range 
Si02,  51-61;  A1203,  16.7-19.6;  CaO,  3.5-6;  MgO,  1-4;  Na2O,  3.8- 
4.5;  K2O,  4.4-5.7.  The  group  should  be  considered  in  connection 
with  absarokite  and  shoshonite.  Journ.  of  Geol.  III.,  937. 

Banatite,  a  name  coined  byB.  v.  Cottain  1865  to  describe  the  dioritic 
rocks  that  are  connected  with  a  series  of  ore  deposits  in  the  Austrian 
province  of  the  Banat.  Accurate  microscopical  study  has  shown  them  to 
be  of  such  varying  mineralogy  that  the  name  has  now  slight  definite  sig- 


GLOSSARY.  127 

nificance.     The  rocks  are  largely  quartz- diorites.      Erzlagerstatten   im 
Banat  und  in  Serbien,  1865. 

Barolite,  Wadsworth's  name  for  rocks  composed  of  barite  or  celes- 
tite.  Rept.  of  State  Geol.  Mich.,  1891-92,  p.  93. 

Barysphere,  a  term  for  the  deep  interior  portions  of  the  earth,  pre- 
sumably composed  of  heavy  metals  or  minerals.  It  is  contrasted  with 
IrtJxJsphere,  the  outer  stony -shell. 

fir  Basalt,  a  word  of  ancient  but  uncertain  etymology  as  stated  on  p.  43. 
It  is  employed  as  a  rock  name  in  its  restricted  sense  for  porphyritic  and 
felsitic  rocks  consisting  of  augite,  olivine  and  plagioclase  with  varying 
amounts  of  a  glassy  base  which  may  entirely  disappear.  In  a  broader 
sense  the  basalt  or  basaltic  group  is  used  to  include  all  the  dark,  basic 
volcanic  rocks,  such  as  the  true  basalts ;  the  nepheline-,  leucite-  and 
melilite-basalts ;  the  augitites  and  limburgites ;  the  diabases,  and  mela~ 
phyres.  The  word  basalt  is  an  extremely  useful  field  name,  as  in  many 
instances  the  finer  discriminations  can  only  be  made  with  the  micro- 
scope. 

r  Basanite,  a  very  old  term,  first  used  as  a  synonym  of  basalt;  also 
formerly  applied  to  the  black,  finely  crystalline  quartzite,  used  by  old- 
time  workers  in  the  precious  metals  as  a  touch-stone  or  test-stone  to  dis- 
tinguish gold  from  brass  by  the  streak.  This  variety  was  often  called 
Lydian  stone  or  lydite.  Basanite  is  now  universally  employed  for  those 
volcanic  rocks,  that  possess  a  porphyritic  or  felsitic  texture  and  that  con- 
tain plagioclase,  augite,  olivine  and  nepheline,  or  leucite,  one  or  both, 
each  variety  being  distinguished  by  the  prefix  of  one,  or  the  other,  or  of 
both  of  the  last  named  minerals.  See  p.  44. 

Basanitoid,  a  term  suggested  by  Bucking  for  basaltic  rocks,  without 
definite  nepheline,  but  with  a  gelatinizing  glassy  base  (H.  Bucking,. 
Jahrb.  d.  k.  k.  preus.  Landesanst,  1882). 

/^"""Base  or  Basis,  is  employed  to  describe  that  part  of  a  fused  rock 
magma  that  in  cooling  fails  to  crystallize  as  recognizable  minerals,  but 
chills  as  a  glass  or  related  amorphous  aggregate.  It  differs  thus  from 
groundmass,  which  is  the  relatively  fine  portion  of  a  porphyritic  rock  as 
distinguished  from  the  phenocrysts. 

^  Basic,  a  general  descriptive  term  for  those  igneous  rocks  that  are 
comparatively  low  in  silica,  55  or  50  per  cent,  is  the  superior  limit.  See 
also  Acidic  and  Medium. 

Bathylite,  a  name  suggested  by  Suess  for  the  vast  irregular  masses  of 
plutonic  rocks  that  have  crystallized  in  depth  and  that  have  only  been 
exposed  by  erosion.  See  p.  12. 

Beerbachite,  a  name  given  by  Chelius  to  certain  small  dikes,  asso- 
ciated with  and  penetrating  large  gabbro  masses,  and  having  themselves 
the  composition  and  texture  of  gabbro.  The  name  was  given  in  the  at- 


128  A   HANDBOOK  OF  ROCKS. 

tempt  to  carry  out  the  questionable  separation  of  the  dike  rocks  from  large 
plutonic  or  volcanic  masses  of  the  same  mineralogy  and  structures. 
Notizbl.  Ver.  Erdkunde  Darmstadt,  1892,  Heft.  13,  p.  i. 

Belonite,  rod  or  club-shaped  microscopic  minerals,  which  usually  oc- 
cur as  embryonic  crystals  in  a  glassy  rock. 

Benches,  a  name  applied  to  ledges  of  all  kinds  of  rock  that  are 
shaped  like  steps  or  terraces.  They  may  be  developed  either  naturally 
in  the  ordinary  processes  of  land-degradation,  faulting,  and  the  like  ;  or 
by  artificial  excavation  in  mines  and  quarries. 

Beresite,  a  name  coined  by  Rose  many  years  ago  for  a  muscovite- 
granite  that  forms  dikes  in  the  gold  district  of  Beresovsk  in  the  Urals. 
It  is,  therefore,  practically  a  synonym  of  aplite,  as  earlier  defined,  but 
some  of  the  beresites  have  since  been  shown  to  be  practically  without 
feldspar,  and  to  form  a  very  exceptional  aggregate  of  quartz  and  musco- 
vite.  (Arzruni,  Zeitsch.  d.  d.  g.,  Gesellsch.,  1885,  865). 
s/  Binary-granite,  a  term  more  or  less  used  in  older  geological  writings 
for  those  varieties  of  granite  that  are  chiefly  quartz  and  feldspar.  See 

P-  31- 

/^     Biotite  is  used  as  a  prefix  to  many  names  of  rocks  that  contain  this 
mica ;  such  as  biotite-andesite,  biotite-gneiss,  biotite-granite,  etc. 

Bituminous,  an  adjective  applied  to  rocks  with  much  organic,  or 
at  least  carbonaceous  matter,  mostly  in  the  form  of  the  tarry  hydrocar- 
bons which  are  usually  described  as  bitumen. 

\^  Blue-ground,  local  miners'  name  for  the  decomposed  peridotite  or 
kimberlite  that  carries  the  diamonds  in  the  South  African  mines. 

Bombs,  masses  of  lava  expelled  from  a  volcano  by  explosions  of 
steam.  They  fall  as  rounded  masses  and  lie  on  the  slopes  of  the  cone,  or 
become  buried  in  tuffs. 

Boninite.  Petersen's  name  for  a  glassy  phase  of  andesite  with  bron- 
zite,  augite  and  a  little  olivine,  from  the  Bonin  Islands,  Japan.  Jahrb. 
Hamburg  Wissensch.  Anst.,  VIII.,  1891.  Compare  sanukite. 

Borolanite,  a  rare  rock  related  to  the  nepheline-syenites  and  de- 
scribed by  Home  and  Teall  from  Borolan,  Sutherlandshire,  Scotland.  It 
has  granitoid  texture,  and  consists  principally  of  orthoclase  and  the 
variety  of  garnet,  called  melanite.  As  accessory  minerals,  biotite,  py- 
roxene, alteration  products  of  nepheline,  sodalite,  titanite,  apatite  and 
magnetite  are  met.  (Trans.  Roy.  Soc.  of  Edinburgh,  1892,  p.  163.) 

Bostonite,  a  name  proposed  by  Hunter  and  Rosenbusch  for  certain 
dikes,  having  practically  the  mineralogical  and  chemical  composition  of 
trachytes  or  porphyries,  except  that  anorthoclase  (and  therefore  soda)  is 
abnormally  abundant  and  dark  silicates  are  few  or  lacking.  They  are 
much  the  same  as  dike-keratophyres  and  were  especially  named  in  carry- 
ing out  the  questionable  separation  of  the  dike-rocks  as  a  distinct  grand 


GLOSSARY,  129 

division  from  the  plutonic  and  volcanic.  The  name  was  suggested  by 
their  supposed  presence  near  Boston,  Mass.,  but  Marblehead,  20  miles  or 
more  distant  is  their  nearest  locality.  They  have  been  since  met  in 
largest  amount  around  Lake  Champlain  and  in  the  neighboring  parts 
of  Canada.  Tscher.  Min.  u.  Petrog.  Mitth.,  1890,  447.  See  also  Bull. 
107,  U.  S.  Geol.  Survey. 

Bouteillenstein,  /.  <?.,  bottlestone,  a  peculiar  green  and  very  pure 
glass,  found  as  rolled  pebbles  near  Moldau,  Bohemia.  It  is  also  called 
moldauite  and  pseudochrysolite,  the  latter  from  its  resemblance  to  oliv- 
ine.  It  is  not  certainly  a  rock,  as  it  may  be  a  prehistoric  slag  or  glass. 

Boulder-clay,  unsorted  glacial  deposits,  consisting  of  boulders, 
clay  and  mud  ;  till,  hardpan. 

Breccia,  a  fragmental  rock  whose  components  are  angular  and  there- 
fore as  distinguished  from  conglomerates  are  not  water-worn.  There  are 
friction  or  fault  breccias,  talus-breccias  and  eruptive  breccias.  The  word 
is  of  Italian  origin.  See  p.  59. 

Broccatello,  an  Italian  word  for  a  brecciated  and  variegated  marble. 
f  Bronzite.  is  often  used  as  a  prefix  to  the  names  of  rocks  containing 
the  mineral.  Rocks  of  the  gabbro  family  are  the  commonest  ones  that 
have  the  prefix. 

Buchnerite,  a  name  proposed  by  Wadsworth  for  those  peridotites, 
terrestrial  and  meteoric,  which  consist  of  olivine,  enstatite  (bronzite)  and 
augite.  The  name  was  given  in  honor  of  Dr.  Otto  Buchner,  an  authority 
on  meteorites.  Lithological  Studies,  1884,  p.  85. 

Buchonite,  a  special  name  given  by  Sandberger  to  a  nepheline-teph- 
rite  that  contains  hornblende.*  Sitzungsberichte  d.  Bed.  Akad.  Wis'., 
July,  1872,  203;  1873,  vi. 

Buhrstone,  a  silicified  fossiliferous  limestone,  with  abundant  cavities 
which  were  formerly  occupied  by  fossil  shells.  Its  cellular  character  and 
toughness  occasioned  its  extensive  use  as  a  millstone  in  former  years. 

c 

Calc-schist,  schistose  rocks,  rich  in  calcite  or  dolomite  and  forming 
intermediate  or  transitional  rocks  between  the  mica-schists  and  crystalline 
limestones.  See  p.  101. 

Camptonite,  a  name  given  by  Rosenbusch  to  certain  dike  rocks, 
having  in  typical  cases  the  mineralogical  composition  of  diorites,  /.  e., 
with  dark  brown  hornblende,  plagioclase,  magnetite,  and  more  or  less 
augite.  They  are  often  porphyritic  in  texture,  and  may  even  have  a 
glassy  groundmass.  Without  the  microscope  camptonites  usually  appear 
as  dark  basaltic  rocks  with  a  few  shining  crystals  of  hornblende  or  augite; 
their  determination  is  essentially  microscopic.  Intimately  associated 


130  A   HANDBOOK  OF  ROCKS. 

with  the  camptonites  of  typical  composition  have  been  found  others  cor- 
responding to  all  varieties  of  basaltic  rocks.  Such  with  prevailing  augite 
have  been  called  augite-camptonite.  The  name  camptonite  is  derived 
from  the  township  of  Campton,  in  the  Pemigewasset  Valley,  N.  H.  The 
original  camptonites  were  discovered  near  Livermore  Falls,  on  the  Pemi- 
gewasset river,  many  years  ago,  /by  O.  P.  Hubbard.  They  were 
microscopically  described  by  G.  W.  Hawes  in  1878,  and  on  this  deter- 
mination Rosenbusch  based  the  name.  They,  or  their  near  relatives, 
have  often  intimate  associations  with  nepheline  syenites.  (See  also,  mon- 
chiquite,  fourchite,  ouachitite.)  Camptonites  are  especially  abundant 
throughout  the  Green  Mountains  and  near  Montreal.  G.  W.  Hawes, 
Amer.  Jour.  Sci.,  1879,  XVII.,  147.  H.  Rosenbusch,  Massigen  Gesteine, 
^87,  333.  Bulletin  107,  U.  S.  Geol.  Surv. 

Carbonolite,  Wadsworth's  name  for  carbonaceous  rocks.  Rept. 
State  Geol.  Mich.,  1891-92,  p.  93. 

JS     Cataclastic,  a  structural  term  applied  to  those  rocks  that  have  suf- 
fered mechanical  crushing  in  dynamic  metamorphism. 

Catawberite,  a  name  given  by  O.  Lieber  to  a  rock  in  South  Caro- 
lina that  is  an  intimate  mixture  of  talc  and  magnetite.  Gangstudien,  III., 

353>  359- 

Catlinite,  a  local  name  in  Minnesota  for  a  red  argillaceous  sandstone, 
presumably  of  Cambrian  age,  that  was  used  by  the  Indians  for  pipe  bowls. 
C.  T.  Jackson,  Amer.  Jour.  Sci.,  1839,  388. 

Catogene,  i.  <?.,  sedimentary  rocks  whose  particles  have  sunk  from 
above  downward. 

Cement,  the  material  that  binds  together  the  particles  of  a  fragmental 
rock.  It  is  usually  calcareous,  siliceous  or  ferruginous.  See  p.  64.  The 
word  is  also  used  in  gold-mining  regions  to  describe  various  consolidated 
fragmental  aggregates,  breccia,  conglomerate  and  the  like  that  are  auri- 
ferous. 

Chalk,  a  marine  calcareous  and  excessively  fine  organic  sediment 
usually  consolidated. 

,/      Chert,  compact   siliceous  rock  formed  of  chalcedonic  or  opaline  sil- 

*     ica,  one  or  both,  and  of  organic  or  precipitated  origin.     See  pp.  74,  80, 

81.     Cherts  often  occur  distributed  through   limestone  affording  cherty 

limestones.     Flint  is  a  variety  of  chert.     Cherts  are  especially  common 

in  the  subcarboniferous  rocks  of  southwest  Missouri. 

Chlorite,  a  general  name  for  the  green  secondary  hydrated  silicates 
containing  alumina  and  iron,  and  derived  especially  from  augite,  horn- 
blende and  biotite.  Chlorite  is  used  as  a  prefix  to  various  names  of 
rocks  that  contain  the  mineral,  such  as  chlorite  schist.  The  name  is  de- 
rived from  the  Greek  word  for  green. 

Chlorophyr,  a  name  given   by  A.  Dumont  to  certain  porphyritic 


GLOSSARY.  131 

quartz  diorites  near  Quenast,  Belgium.     See  Delesse,  Bull.  Soc.  Geol.  de 
France,  1850,  315. 

Clastic,  descriptive  term  applied  to  rocks  formed  from  the  fragments 
of  other  rocks  ;  fragmental. 

Clay,  general  name  for  the  fine  aluminous  sediments  that  are  plastic. 
Though  usually  soft,  they  may  be  so  hard  as  to  need  grinding  before  the 
plasticity  manifests  itself,  as  in  numerous  fire  clays.  See  p.  67. 

Clay  slate,  metamorphosed  clay,  with  new  cleavages  developed  by 
pressure  and  shearing.  The  term  is  used  in  distinction  to  mica-slate,  and 
other  slaty  rocks.  See  p.  108. 

Claystone-porphyry,  an  old  and  somewhat  indefinite  name  for 
those  porphyries  whose  naturally  fine  groundmass  is  more  or  less  kaolin- 
ized,  so  as  to  be  soft  and  earthy,  suggesting  hardened  clay. 

Clinkstone.     See  phonolite. 

\       Composite  dike,  a  dike  formed  by  two  intrusions  of  different  ages 
\into  the  same  fissure  (W.  Judd,  Quar.  Jour.  Geol.  Soc.,  1893,  536). 

Concretions,  spheroidal  or  discoid  aggregates  formed  by  the  segre- 
gation and  precipitation  of  some  soluble  mineral  like  quartz  or  calcite 
around  a  nucleus,  which  is  often  a  fossil. 

Cone-in-cone,  a  curious  structure,  occasionally  met  in  clay  rocks, 
whereby  two  opposing  and  interlocking  sets  of  cones  or  pyramids  are  de- 
veloped, with  their  axes  parallel  and  their  bases  in  approximately  paral- 
lel surfaces. 
/'^Conglomerate,  consolidated  gravel.     See  p.  62. 

Consanguinity,  a  term  used  by  Iddings  to  describe  the  genetic  rela- 
tionship of  those  igneous  rocks  which  are  presumably  derived  from  a 
common,  parent  magma.  See  p.  57,  and  Bull.  Phil.  Soc.  Washington 
XI^  89. 

^  Contact,  the  place  or  surface  where  two  different  kinds  of  rocks  come 
together.  Although  used  for  sedimentary  rocks,  as  the  eontact  between  a 
limestone  and  sandstone,  it  is  yet  more  especially  employed  as  between 
igneous  intrusions  and  their  walls.  The  word  is  of  wide  use  in  western 
mining  regions  on  account  of  the  frequent  occurrence  of  ore-bodies  along 
contacts.  On  contact- metamorphism,  see  pp.  85-92. 

y      Cordierite,  a  synonym  of  iolite  or  dichroite,  employed  as  a  prefix  to 
those  rocks  that  contain  the  mineral,  as  cordierite-gneiss. 

Cornubianite,  a  name  coined  by  Boase  from  the  classic  name  for 
Cornwall,  England,  to  describe  a  contact  hornfels,  consisting  of  anda- 
lusite,  mica  and  quartz.  It  was  proposed  as  a  substitute  for  an  earlier 
but  indefinite  term  proteolite.  Bonney  suggests  restricting  cornubianite 
to  tourmaline  hornfels.  Quar.  Jour.  Geol.  Soc.,  1886,  104 
I/  Corrasion,  geological  term  for  the  wearing  away  of  rocks  by  grit 
suspended  in  moving  water  or  air;  to  be  distinguished  from  erosion. 


132  A    HANDBOOK  OF  ROCKS. 

Corroded  crystals,  phenocrysts  that  after  crystallization  are  more  or 
less  reabsorbed  or  fused  again  into  the  magma. 

Corsite,  a  name  applied  by  Zirkel  to  the  orbicular  or  spheroidal 
diorite  from  Corsica;  synonym  of  napoleonite.  Lehrb.  d.  Petrographie, 
1866,  II.,  133,  320. 

Cortlandite,  a  special  name  given  by  G.  H.  Williams  to  a  peridotite 
that  consists  chiefly  of  hornblende  and  olivine  and  that  occurs  in  the  so- 
called  Cortland  series  of  igneous  rocks  in  the  township  of  Cortland,  just 
south  of  Peekskill,  on  the  Hudson  River.  This  rock  had  been  previously 
called  hudsonite  by  E.  Cohen,  a  name  rejected  by  Williams  because 
already  used  for  a  variety  of  pyroxene.  Amer.  Jour.  Sci.,  Jan.,  1886,  30. 

Corundolite,  Wadsvvorth's  name  for  rocks  composed  of  corundum  or 
emery.  Rept.  State  Geol.  Mich.,  1891-92,  p.  92. 

Crenitic,  a  word  derived  from  the  Greek  for  spring,  and  especially 
used  by  T.  S.  Hunt  for  those  rocks  which  were  thought  by  him  to  have 
come  to  the  surface  in  solution  and  to  have  been  precipitated.  He  used 
the  so-called  "  crenitic  hypothesis  "  to  explain  certain  schists  whose  feld- 
spars were  supposed  to  have  been  originally  zeolites,  but  his  views  have 
received  slight,  if  any,  support.  Proc.  Roy.  Soc.  Canada,  Vol.  II.,  Sec. 
III.,  1884.  Reprint,  p.  25.  Crenitic  is  also  used  by  W.  O.  Crosby  to 
describe  those  mineral  veins  which  have  been  deposited  by  uprising 
springs.  Technology  Quarterly,  April,  1894,  p.  39. 

Cross-bedding,  or  Cross-stratification,  descriptive  terms  applied 
to  those  minor  or  subordinate  layers  in  sediments  that  are  limited  to 
single  beds,  but  that  are  inclined  to  the  general  stratification.  They  are 
caused  by  swift  local  currents,  deltas,  or  swirling  wind-gusts,  and  are 
especially  characteristic  of  sandstones,  both  aqueous  and  eolian.  See  pp. 
64,  65. 

Crustification,  the  English  equivalent  of  a  term  suggested  by  Posep- 
ny  for  those  deposits  of  minerals  and  ores  that  are  in  layers  or  crusts  and 
that,  therefore,  have  been  distinctively  deposited  from  solution.  Trans. 
Amer.  Inst.  Min.  Eng.,  XXIII. ,  207,  1893. 

Crypto-crystalline,  formed  of  crystals  of  unresolvable  fineness,  but 
not  glassy.  A  submicroscopical  crystalline  aggregate. 

Crystallites.  The  term  is  most  properly  applied  only  to  small,  rudi- 
mentary or  embryonic  crystals,  not  referable  to  a  definite  species,  but  it 
is  also  used  as  a  general  term  for  microscopic  crystals. 

Cumberlandite,  a  name  derived  from  Cumberland  Hill,  R.  I.,  pro- 
posed by  Wadsworth  for  the  ultra-basic  igneous  rock,  forming  the  hill. 
It  is  an  aggregate  of  titaniferous  magnetite,  plagioclase,  olivine  and  sec- 
ondary minerals,  but  contains  from  40-45  per  cent,  iron  oxides  and 
about  10  per  cent.  TiO2.  "  Lithological  Studies,  1884.  j* 

Cumulites,  Vogelsang's  name  for  spherulitic  aggregates  of  globulites.. 
Die  Krystalliten,  1875. 


GLOSSARY.  133 

Cuselite,  Rosenbusch's  name  for  a  peculiar  variety  of  augite-porphy- 
rite  from  Cusel,  in  the  Saar  basin.  Massige  Gest.,  503,  1887. 

D 

Dacite,  quartz-bearing  andesites.  The  name  was  suggested  by  the 
ancient  Roman  province  of  Dacia,  now  in  modern  Hungary.  See  p.  40. 

Damourite-schist,  a  micaceous  schist  whose  micaceous  mineral  is 
damourite.  Much  the  same  as  hydro-mica  schist.  Seep.  101. 

Desmosite,  a  banded  contact  rock  developed  from  shales  and  slates 
by  intrusions  of  diabase.  Compare  spilosite  and  adinole.  See  Zincken, 
Karsten  und  v.  Dechen's  Archiv.,  XIX.,  584,  1845. 

Detritus,  a  general  name  for  incoherent  sediments  produced  by  the 
wear  and  tear  of  rocks  through  the  various  geological  agencies.  The 
name  is  from  the  Latin  for  "  worn." 

Devitrification,  the  process  by  which  glassy  rocks  break  up  into 
definite  minerals.  The  latter  are  usually  excessively  minute,  but  are 
chiefly  quartz  and  feldspars. 

Diabase,  igneous  rocks,  in  sheets  or  dikes,  consisting  essentially  of 
plagioclase,  augite  and  magnetite,  with  or  without  olivine,  and  possessing 
ophitic  tenure.  See  p.  44.  The  word  has  had  a  somewhat  variable  sig- 
nificance during  its  history,  but  with  the  final  exit  of  the  time-element  in 
the  classification  of  igneous  rocks  its  present  significance  is  generally  ac- 
cepted  as  above  given.  '  '  -7 

^  Diallage,  the  variety  of  monoclinic  pyroxene  that,  in  addition  to  the 
prismatic  cleavages,  has  others  parallel  to  the  vertical  pinacoids.  Used  also 
as  a  prefix  to  many  rocks  containing  the  mineral. 

/^Diatomaceous  earth,  rocks  essentially  formed  of  the  abandoned 
frustules  of  the  microscopic  organisms  called  diatoms. 

Dichroite,  see  under  cordierite. 

r  Dikes,  spelled  also  dykes,  intrusions  of  igneous  rocks  in  fissures: 
not  to  be  confused  with  veins  which  are  precipitated  from  solution. 

Diluvium,  a  name  formerly  applied  to  the  unsorted  and  sorted  de- 
posits of  the  Glacial  period,  as  contrasted  with  the  later  water  sorted 
alluvium,  which  see. 

/  Diorite,  a  granitoid  rock  consisting  essentially  of  plagioclase  and 
hornblende.  More  or  less  biotite  is  usually  present,  which  may  even  re- 
place the  hornblende,  yielding  mica-diorite ;  augite  also  often  appears. 
Acidic  varieties  with  quartz  are  called  quartz  diorites.  See  pp.  47,  48. 
Diorite  is  often  used  as  a  prefix  for  porphyritic  or  other  rocks  related  to 
diorite.  The  name  is  from  the  Greek  to  distinguish  and  was  given  by 
Hauy  in  1822. 

Dipyr,  a  variety  of  sca$>olite,  aften  used  as  a  prefix  to  the  names  of 
rocks  that  contain  the  mineral. 


134  A   HANDBOOK  OF  ROCKS. 

Disthene,  synonym  of  cyanite,  sometimes  used  as  a  prefix  in  rock 
names. 

Ditroite,  a  nepheline  syenite  from  Ditro  in  Hungary,  especially  rich 
in  blue  sodalite.  See  p.  37. 

Dolerite,  coarsely  crystalline  basalts.  The  word  has  had  a  somewhat 
variable  meaning  during  its  history  and  among  different  peoples.  The 
English  use  it  interchangeably  with  diabase ;  indeed  the  definitions  given 
here  justify  this  usage,  except  that  the  ophitic  texture  of  diabase  is  not 
essential  to  this  definition  of  dolerite.  But  the  ophitic  texture  is  more 
of  a  microscopic  feature  than  megascopic.  Dolerite  is  from  the  Greek 
for  deceptive,  and  was  given  by  Hauy  in  allusion  to  its  application  to 
later  rocks  that  could  not  be  distinguished  from  older  greenstones.  The 
name  is  a  long  standing  indictment  of  the  time  element  in  the  classifica- 
tion of  igneous  rocks. 

Dolomite  is  applied  to  those  rocks  that  approximate  the  mineral  dolo- 
mite in  composition.  Named  by  Saussure,  after  Dolomieu,  an  early 
French  geologist.  See  p.  72. 

Dolomitization  or  Dolomization,  the  process  whereby  limestone 
becomes  dolomite  by  the  substitution  of  rnagnesian  carbonate  for  a  por- 
tion of  the  original  calcium  carbonate.  If  the  MgCO3  reaches  45.65  per 
cent,  there  is  great  shrinkage  in  bulk,  leading  to  the  development  of 
porosity  and  cavities  up  to  1 1  per  cent,  of  the  orignal  rock. 

Domite,  a  more  or  less  decomposed  trachyte  from  the  Puy  de  Dome 
in  the  French  volcanic  district  of  the  Auvergne.  The  typical  domite 
contains  oligoclase  and  is  impregnated  with  hematite. 

Drift,  a  general  name  for  the  unsorted  deposits  of  the  glacial  period. 
Where  subsequently  worked  over  by  water  they  are  called  modified  drift. 

Dunite,  a  member  of  the  peridotite  group  that  consists  essentially  of 
olivine  and  chromite.  It  was  named  from  the  Dun  Mountains  in  New 
Zealand,  the  original  locality,  but  it  also  occurs  in  North  Carolina.  The 
name  was  given  by  V.  Hochstetter  in  1859.  Geol.  v.  Neu  Seeland,  218, 
1864. 

Durbachite,  a  name  give  to  a  basic  development  at  the  outer  border 
of  a  granite  intrusion  in  Baden.  It  has  the  general  composition  of  mica 
syenite.  The  name  was  given  by  Sauer,  Mitth.  d.  grossh.  bad.  Geol. 
Landesanstalt,  II.,  233. 

Dykes,  see  dikes. 

Dynamometamorphism,  a  general  term  for  those  metamorphic 
changes  in  rocks  that  are  produced  by  mechanical  as  distinguished  from 
chemical  processes,  but  the  former  are  seldom  unattended  by  the  latter. 
See  p.  93. 

Dysyntribite,  a  name  given  by  C.  U.  Shepard,  A.  A.  A.  S.,  1851, 
311,  to  a  mineral  or  rock  in  St.  Lawrence  Co.,  N.  Y.,  which  is  a  hydrated 


GLOSSARY.  135 

silicate  of  aluminium  and  potassium,  and  is  related  to  pinite;  the  name 
means  hard  to  crush.  Compare  parophite.  See  also,  Smith  and  Brush, 
Amer.  Jour.  Sci.,  II.,  XVI.,  50,  and  C.  H.  Smyth,  Jour,  of  Geol.,  II., 
678,  1894. 


Eclogite,  a  more  or  less  schistose  metamorphic  rock,  consisting  of  a 
light-green  pyroxene  (omphacite),  actinolite  (var.  smaragdite)  and  gar- 
net. Scarcely  known  in  America.  See  p.  105  and  anal.  6,  p.  103.  The 
name  is  from  the  Greek  to  select,  in  reference  to  its  attractive  appearance. 

Effusive,  a  name  applied  to  those  rocks  that  have  poured  out  in  a 
molten  state  on  the  surface  and  have  there  crystalized,  /,  ^.,  volcanic 
rocks.  See  p.  13. 

Elaeolite,  or  EJe_oJliie,  a  name  formerly  current  for  the  nepheline  of 
pre-tertiary  rocks.  It  is  best  known  in  the  rock-name  eleolite-syenite, 
a  synonym  of  nepheline-syenite,  which  is  preferable.  See  nepheline- 
syenite. 

Elvan,  Cornish  name  for  dikes  of  quartz-porphyry  or  of  granite-por- 
phyry. 

Endomorphic,  used  as  a  descriptive  adjective  for  those  phases  of 
contact-metamorphism  that  are  developed  in  the  intrusion  itself.  It  is 
synonymous  with  internal  as  used  on  p.  87. 

Enstatite,  the  variety  of  orthorhombic  pyroxene  with  less  than  5  per 
cent.  FeO.  It  is  largely  used  as  a  prefix  to  the  names  of  rocks  that  con- 
tain the  mineral. 

Epidiorite,  a  name  applied  to  dikes  of  diabase,  whose  augite  is  in 
part  altered  to  green  hornblende.  The  name  was  coined  before  it  was 
understood  that  the  hornblende  was  secondary  in  this  way.  It  was  first 
applied  by  Giimbel  in  1879  to  a  series  of  narrow  dikes  that  cut  Cam- 
brian and  Ordovician  strata  in  the  Fichtelgebirge.  The  name  empha- 
sizes their  later  age  than  the  typical  pre-Cambrian  diorites,  but  its  sig- 
nificance has  been  expanded  in  later  years. 

Epidosite,  rocks  largely  formed  of  epidote.  The  epidote  seems 
generally  to  be  produced  by  the  reactions  of  feldspars  and  bisilicates 
upon  each  other  during  alteration. 

Epidote,  the  name  of-  the  mineral  is  often  prefixed  to  to  the  names  of 
rocks  containing  it.     As  a  rule,  the  presence  of  epidote  indicates  the  ad- 
vance of  alteration. 
f'  Erosion,  geological  term  for  the  process  of  the  removal  of  loose  ma- 

terials in  suspension  in  running  water  or  in  wind. 

^  Eruptive,   the  name  ought  properly  to  be  only  applied  to  effusive 
or  volcanic  rocks,  but  it  is  often  used  as  a  synonym  of  igneous. 

Eucrite,  a  name  given  by  G.  Rose  to  rocks  and  meteorites  that  con- 


136  A   HANDBOOK  OF  ROCKS. 

sist  essentially  of  anorthite  and  augite.     The  term  is  practically  obsolete. 
Pogg.  Annalen,  1835,  I.,  i. 

Eudyalite,  the  name  of  the  mineral  is  sometimes  prefixed  to  the 
rare  nepheline  syenites  that  contain  it. 

Eulysite,  a  name  given  by  Erdmann  to  rocks  interlaminated  with  the 
gneisses  of  Sweden,  and  consisting  of  olivine,  green  pyroxene  and  gar- 
net. Neues  Jahrb.  1849,  837. 

Euphotide,  the  name  chiefly  used  among  the  French  for  gabbro.  It 
was  given  by  Hauy,  and  is  derived  from  the  Greek  words  for  well  and 
light,  in  allusion  to  its  pleasing  combination  of  white  and  green. 

Eurite,  used  among  the  French  as  a  synonym  of  felsite,  but  also  ap- 
plied to  compact  rocks  chiefly  feldspar  and  quartz,  such  as  some  granu- 
lites.  The  name  was  first  given  by  Daubuisson  to  the  groundmass.es  of 
porphyries,  because  of  their  easy  fusibility  as  compared  with  hornstone 
or  flint. 

Eutaxitic,  a  general  name  for  banded  volcanic  rocks.  The  banding 
is  due  to  the  parallel  arrangement  of  portions  of  the  rock  that  are  con- 
trasted either  in  mineralogy  or  texture. 

*      Exomorphic,  a  descriptive  term  for  those  changes  produced  by  con- 
tact-metamorphism  in  the  wall  rock  of  the  intrusion ;  the  antithesis  of 
endomorphic.     It  is  synonymous  with  external  as  used  in  p.  88. 
/^    Extrusive,  synonym  of  effusive,  much  used  in  America. 


Feldspar,  the  name  of  the  mineral  is  often  prefixed  to  the  names  of 
those  rocks  that  contain  it,  as  feldspar-porphyry,  feldspar-basalt,  etc. 

Feldspathoids,  silicates  of  alumina  and  an  alkali  or  alkaline  earth, 
that  are  practically  equivalent  to  feldspars  in  their  relations  in  rocks. 
The  principal  ones  are  nepheline,  leucite  and  melilite,  but  sodalite, 
nosean  arid  hauyne  could  perhaps  be  also  considered  such,  although  their 
composition  varies  from  the  above. 

Felsite,  the  word  was  first  applied  in  1814  by  Gerhard,  an  early  ge- 
ologist, to  the  fine  groundmasses  of  porphyries.  These  were  recognized 
to  be  fusible  as  distinguished  from  hornstone,  which  they  resembled  (com- 
pare eurite).  Felsite  is  now  especially  used  for  those  finely  crystalline 
varieties  of  quartz-porphyries,  porphyries  or  porphyrites  that  have  few  or 
no  phenocrysts,  and  that  therefore  give  but  slight  indications  to  the  un- 
aided eye  of  their  actual  mineralogical  composition.  The  microscope 
has  shown  them  to  be  made  up  of  microscopic  feldspars,  quartzes  and 
glass.  Petrosilex  has  been  used  as  a  synonym.  See  p.  24. 

Felsitic  has  been  employed  as  a  megascopic  term  in  the  preceding 
pages  to  describe  those  textures  which  are  characteristic  of  felsites,  /".  e., 
micro-crystalline,  but  without  phenocrysts.  See  p.  15.  It  is  often  used 


GLOSSARY.  137 

also  to  describe  the  groundmasses  of  truly  porphyritic  rocks,  that  are 
micro-crystalline,  but  clearly  not  glassy.  In  this  sense  we  have  felsite 
porphyry,  felso-liparite,  felso-dacite,  etc. 

Felspar,  the  current  spelling  of  feldspar  among  the  English.  It  is 
based  on  an  old  typographical  error  in  Kirwan's  Mineralogy,  I.,  317, 
1794,  now,  however,  firmly  established  in  general  usage. 

Ferrite,  microscopic  crystals  of  iron  oxide. 

Ferrolite,  Wadsworth's  name  for  rocks  composed  of  iron  ores.  Rept. 
State  Geol.  Mich.,  1891-92,  p.  92. 

Fibrolite,  synonym  of  sillimanite,  and  sometimes  used  as  a  prefix  to 
rock  names. 

Fiorite,  siliceous  sinter,  named  from  Mt.  Santa  Fiora,  in  Tuscany. 

Firn,  Swiss  name  for  the  granular,  loose  or  consolidated  ice  of  the 
high  altitudes  before  it  forms  glacial  ice  below. 

Flaser-structure,  a  structure  developed  in  granitoid  rocks  and  espe- 
cially in  gabbros  by  dynamic  metamorphism.  Small  lenses  of  granular 
texture  are  set  in  a  scaly  aggregate  that  fills  the  interstices  between  them. 
It  appears  to  have  been  caused  by  shearing  that  has  crushed  some  por- 
tions more  than  others  and  that  has  developed  a  kind  of  rude  flow-struc- 
ture. 

Flint,  a  compact  and  crypto- crystalline  aggregate  of  chalcedonic  and 
opaline  silica.  Chert  and  hornstone  are  synonyms.  See  pp.  74,  80. 

Float,  a  term  much  used  among  Western  miners  for  loose  surface  de- 
posits, which  are  usually  somewhere  near  their  parent  ledges. 
'  ^/"^Flow-structure,  a  structure  due  to  the  alignment  of  the  minerals  or 
inclusions  of  an  igneous  rock  so  as  to  suggest  the  swirling  curves,  eddies 
and  wavy  motions  of  a  flowing  stream.  It  is  caused  by  the  chilling  of  a 
flowing  lava  current.  Fluxion-structure  is  synonymous. 

Foliation,  the  banding  or  lamination  of  metamorphic  rocks  as  dis- 
tinguished from-  the  stratification  of  sediments. 

Forellenstein,  a  variety  of  olivine-gabbro,  consisting  of  plagioclase, 
olivine  and  more  or  less  pyroxene.  The  dark  silicates  are  so  arranged  in 
the  lighter  feldspar  as  to  suggest  the  markings  of  a  trout.  (German,  Forelle.) 

Formation,  as  defined  and  used  by  the  U.  S.  Geological  Survey,  it 
is  a  large  and  persistent  stratum  of  some  one  kind  of  rock.  It  is  also 
loosely  used  for  any  local  and  more  or  less  related  group  of  rocks.  In 
Dana's  Geology  it  is  used  for  the  groups  of  related  strata  that  were  formed 
in  a  geological  period. 

Fourchite,  a  name  proposed  by  J.  Francis  Williams  for  those  basic 
dike  rocks  that  consist  essentially  of  augite  in  a  glassy  groundmass,  /.  ^., 
dike-augitites.  The  name  was  suggested  by  Fourche  Mountain,  Ark., 
where  they  are  abundant.  Ann.  Rep.  Geol.  Sur.,  Ark.,  1890,  II.,  107. 

Foyaite,  a  name  originally  applied  to  the  nepheline  syenite,  with 


138  A   HANDBOOK  OF  ROCKS. 

supposed  hornblende,  of  Mt.  Foya,  in  the  Monchique  range  of  Portugal. 
Although  the  hornblende  has  since  proved  to  be  augite  and  segirine  the 
name  foyaite  is  still  employed  for  nepheline  syenite  with  hornblende. 
See  p.  37. 

Fragment al,  descriptive  term  for  the  rocks  formed  from  fragments  of 
preexisting  rocks,  such  as  sandstones,  breccias,  clastic. 

Fraidronite,  a  name  used  by  early  French  geologists  for  a  variety  of 
minette. 

Freestone,  a  quarryman's  name  for  those  sandstones  that  submit 
readily  to  tool  treatment. 

Fruchtschiefer,  German  name  for  a  variety  of  spotted  contact  schists 
in  the  outer  zone  of  the  aureole.  See  p.  89. 

Fuller's  earth,  a  fine  earth,  resembling  clay,  but  lacking  plasticity. 
It  is  much  the  same  chemically  as  clay,  but  has  a  decidedly  higher  per- 
centage of  water. 

Fulgurite,  little  tubes  of  glassy  rock  that  have  been  fused  from  all  sorts 
of  rock  by  lightning  strokes.  They  are  especially  frequent  in  exposed  crags 
on  mountain  tops.  The  name  is  derived  from  the  Latin  for  thunderbolt. 

o 

r  Gabbro,  an  Italian  word  formerly  used  for  a  rock  composed  of  serpen- 
tine and  diallage.  It  was  later  applied  to  igneous  rocks,  of  granitoid 
texture,  consisting  of  plagioclase  and  diallage,  but  as  now  used  any  mono- 
clinic  pyroxene  may  be  present  with  or  without  diallage.  As  the  name 
of  a  group  it  includes  those  rocks  with  plagioclase  and  orthorhombic 
pyroxene  as  well,  and  even  the  peridotites  from  their  close  geological 
connection  with  the  gabbros  may  conveniently  be  embraced.  Although 
of  the  same  mineral  composition  with  gabbro,  yet  the  peculiar  ophitic 
texture  of  diabase  may  be  remarked.  Intermediate  types  have  even  been 
called  gabbro-diabase.  See  p.  50.  A  full  review  of  the  meaning  and 
history  of  gabbro,  by  W.  S.  Bayley,  will  be  found  in  Jour,  of  Geology  I., 
435,  Aug.,  1893. 

Gabbro-diorite,  gabbro  with  hornblende  which  may,  in  fact,  be 
secondary  after  augite.  Intermediate  rocks  between  true  gabbros  and 
diorites. 

Ganggesteine,  German  for  dike  rocks. 

Garnet-rock,  a  rock  composed  essentially  of  garnets. 

s^  Generations  of  minerals  in  an  igneous  rock  refer  to  the  groups  of 
individuals  that  crystallize  out  at  a  definite  period  and  in  a  more  or  less 
definite  succession  during  cooling.  The  same  species  may  have  one, 
two  or  very  rarely  three  generations.  See  p.  17. 

y'  Geodes,  hollow,  rounded  boulders  lined  with  crystals  projecting  in- 
ward from  the  walls. 


GLOSSARY.  139 

Geyserite,  siliceous  deposits  from  a  geyser.     See  p.  80. 

Gieseckite-porphyry,  a  nepheline  porphyry  from  Greenland,  whose 
nepheline  phenocrysts  are  altered  to  the  aggregate  of  muscovite  scales, 
which  was  called  gieseckite  under  the  impression  that  it  was  a  new 
mineral.  Liebenerite  porphyry  is  the  same  thing  from  Predazzo,  in  the 
Tyrol. 

Glass,  the  amorphous  result  of  the  quick  chill  of  a  fused  lava.  See 
pp.  20,  21,  22,  and  for  glassy  texture,  p.  14. 

Glauconite,  the  green  silicate  of  iron  and  potassium  that  is  important 
in  many  green  sands.  See  p.  69. 

£/  Glaucophane,  a  blue  soda-amphibole  found  especially  in  certain  rare 
schists.     See  pp.  103,  105. 

Gneiss,  a  laminated  or  foliated  granitoid  rock  that  corresponds  in 
mineralogical  composition  to  some  one' of  the  plutonic  rocks.  The  name 
originated  among  the  Saxon  miners.  See  p.  105. 

Granite,  in  restricted  signification  is  a  granitoid  igneous  rock  consist- 
ing of  quartz,  orthoclase,  more  or  less  oligoclase,  biotite  and  muscovite, 
but  it  is  widely  used  in  a  more  general  sense.  The  first  three  may  also  be 
combined  with  either  of  the  micas  alone,  with  hornblende  or  with  augite. 
In  its  technical  applications  as  a  name  of  a  building  stone  it  is  used  for  al- 
most any  crystalline  rock  composed  of  silicates,  as  contrasted  with  sand- 
stones, slates,  limestones  and  marbles.  It  is  a  very  old  term.  See  p.  31. 

Granitelle,  a  granite  with  comparatively  little  mica,  so  that  it  con- 
sists almost  entirely  of  quartz  and  feldspar;  binary  granite.  It  has  been 
also  used  by  R.  D.  Irving  for  augite-granite.  U.  S.  Geol.  Surv.,  Mono- 
graph V.,  p.  115. 

Granite-porphyry,  practically  a  quartz-porphyry  with  a  coarsely 
crystalline  groundmass  ;  an  intermediate  rock  between  granites  and  typ- 
ical quartz-porphyries,  having  the  same  minerals  as  the  former,  but  be- 
ing porphyritic  like  the  latter.  The  chief  phenocrysts  are,  however, 
feldspars.  See  pp.  25,  32. 

Granitite,  a  special  name  for  biotite-granite.  It  is  much  the  com- 
monest of  the  granites. 

Granitoid,  used  in  preceding  pages  as  a  textural  term  to  describe 
those  igneous  rocks  which  are  entirely  composed  of  recognizable  minerals 
of  approximately  the  same  size.  It  was  suggested  by  granite,  the  most 
familiar  of  the  rocks  which  show  this  characteristic.  See  p.  13.  In  the 
granitoid  texture  each  kind  of  mineral  appears  in  but  one  generation,  and 
the  individuals  seldom  have  crystal  boundaries. 

Granophyric,  a  descriptive  term  used  in  connection  with  micro- 
scopic study,  to  describe  those  groundmasses  in  quartz -porphyries  in 
which  the  quartz  and  feldspar  crystals  have  simultaneously  crystallized  so 
-as  to  mutually  penetrate  one  another.  The  several  parts  of  one  indi- 


140  A   HANDBOOK  OF  ROCKS. 

vidual,  though  separated  from  one  another,  extinguish  together  be- 
tween crossed  nicols.  See  also  micro-pegmatitic,  micro-perthitic,  micro- 
poicilitic,  and  micro-granitic. 

^  Granulite,  properly  speaking  a  finely  crystalline,  laminated  meta- 
morphic  rock  consisting  essentially  of  orthoclase,  quartz  and  garnet,  but 
having  also  at  times  cyanite,  hornblende,  biotite  or  augite.  It  is  best 
developed  in  the  mountains  of  Saxony.  Sometimes  the  name  is  less  cor- 
rectly used  for  muscovite  granite,  or  for  granites  containing  little  else 
than  quartz  and  feldspar.  See  p.  98. 

Graphite,  the  name  of  the  mineral  is  often  prefixed  to  the  names 
of  rocks  containing  it,  as  graphite-gneiss,  graphite-schist,  etc. 
//  Graywacke,  an  old  name  of  loose  signification  but  chiefly  applied  to- 
metamorphosed  shaly  sandstones  that  yield  a  tough,  irregularly  breaking 
rock,  different  from  slate  on  the  one  hand  and  from  quartzite  on  the 
other. 

Greenschists,  chlorite   schists,   which  may,  however,   be  of  quite 
diverse  origin.     See  p.  104. 

Greenstone,  an  old  field  name  for  those  compact  igneous  rocks  that 
have  developed  enough  chlorite  in  alteration  to  give  them  a  green  cast. 
They  are  mostly  diabases  and  diorites.  Greenstone  is  partially  synony- 
mous with  trap.  It  is  often  used  as  a  prefix  to  other  rock  names. 
j/  Greisen,  a  granitoid  but  often  somewhat  cellular  rock,  composed  of 
quartz  and  muscovite  or  some  related  mica,  rich  in  fluorine.  It  is  the 
characteristic  mother  rock  of  the  ore  of  tin,  cassiterite,  and  is  in  most 
cases  a  result  of  the  contact  action  of  granite  and  its  evolved  mineralizers. 
See  p.  92. 

Grit,  coarse  sandstone. 

Grorudite,  Brogger's  name  for  a  porphyritic  dike  rock  from  Grorud, 
near  Christiania,  Norway.  The  phenocrysts  are  microcline  and  aegirine  ; 
the  groundmass  consists  of  rectangular  orthoclase,  quartz  and  aegirine. 
It  is  a  variety  of  granite  porphyry.  Zeitsch.  f.  Krys.,  XVI.,  65. 
/>  Groundmass,  the  relatively  finely  crystalline  or  glassy  portion  of  a 
porphyritic  rock  as  contrasted  with  its  phenocrysts.  Not  to  be  confused 
with  basis,  as  will  be  seen  by  referring  to  the  latter.  On  groundmass, 
see  ^14. 

/  £  /?  -  Gumbo,  a  name  current  in  Western  and  Southern  States  for  those 
soils  that  yield  a  sticky  mud  when  wet. 


Halleflinta,  a  Swedish  name  for  dense,  compact  metamorphic  rocks* 
consisting  of  microscopic  quartz  and  feldspar  crystals,  with  occasional 
phenocrysts  and  sometimes  hornblende,  chlorite,  magnetite  and  hema- 
tite. They  are  associated  with  gneisses,  but  are  of  obscure  origin. 


^ 


GLOSSARY.  141 

Haloidite,  Wadsworth's  name  for  rock-salt.  Kept.  State  Geol.  Mich., 
1891-92,  p.  92. 

Hard-pan.    See  boulder-clay. 

Harzburgite,  a  name  proposed  by  Rosenbusch  for  those  peridotites 
that  consist  essentially  of  olivirie  and  enstatite  or  bronzite.  Mass.  Gest., 
1887,  269.  Saxonite  was  earlier  proposed  by  Wadsworth  (1884)  for  the 
same  rock,  and  has  priority. 

Hauyne,  the  name  of  the  mineral  is  often  prefixed  to  the  names  of 
those  rocks  that  contain  it,  as  hauyne-basalt,  hauyne-trachyte,  etc. 

Hedrumite,  a  name  proposed  by  Brogger  for  certain  syenitic  rocks 
that  are  poor  or  lacking  in  nepheline,  but  that  have  a  trachytic  texture. 
Zeitschr.  Krys.,  XVI.,  40.  They  are  not  yet  (1895)  fully  described. 

Hemithrene,  Brogniart's  name,  current  among  the  French,  for  cer- 
tain dioritic  rocks  that  have  a  large  amount  of  calcite,  presumably  an 
alteration  product. 

^Holocrystalline,  a  textural  term  applied  to  those  rocks  that  consist 
entirely  of  crystallized  minerals  as  distinguished  from  those  with  more  or 
less  glass. 

Hornblende,  the  name  of  the  mineral  is  prefixed  to  many  rock 
names. 

Hornblendite,  a  granitoid  igneous  rock  consisting  essentially  of  horn- 
blende, analogous  to  pyroxenite.  See  p.  51. 

Hornfels,  dense,  compact  rocks  produced  from  slates  by  the  contact 
action  of  some  igneous  intrusion,  especially  granite.  Various  microscopic 
minerals  are  developed  in  them.  See  p.  88. 

Hornstone,  synonym  of  flint  and  chert. 

ses,  a  miner's  term  for  fragments  of  wall  rock  included  in  a  vein. 

Hudsonite.    See  cortlandite. 

Hyaline,  synonym  of  glassy  and  often  prefixed  to  the  name  of  vol- 
canic rocks  to  signify  a  glassy  development,  as  hyalo  rhyolites. 

Hyalomelane,  Hausmann's  name  for  basaltic  glass.  The  word  is 
derived  from  the  Greek  for  black  glass. 

Hydato,  a  syllable  prefixed  to  lithologieal  terms  to  indicate  an  origin 
through  aqueous  processes. 

Hydatopneumatolithic,  a  term  used  in  discussions  of  certain  ore 
deposits  to  describe  their  origin  through  the  agency  of  water  and  vapors. 

Hyperite,  used  in  Sweden  loosely  for  the  rocks  of  the  gabbro  family, 
and  in  a  restricted  sense  for  olivine-norite. 

Hypersthenite,  a  somewhat  obsolete  name  for  norite. 

Hysterogenite,  Posepny's  term  for  mineral  deposits  of  latest  forma- 
tion, such  as  those  from  the  debris  of  other  rocks.  Trans.  Amer.  Inst. 
Min.  Eng.,  XXIII.,  211.  Compare  idiogenite,  xenogenite. 

Hysteromorphous,  a  term  suggested  by  Posepny  for  those  ore  de- 


142  A    HANDBOOK  OF  ROCKS. 

posits  that  have  been  formed  by  the  chemical  and  mechanical  influences- 
of  the  surface  region  from  some  other  original  deposits.  Trans.  Amer. 
Inst.  Min.  Eng.,  XXIII.,  331,  1893. 


Idiogenites,  a  term  suggested  by  Posepny  to  describe  those  ore 
deposits  which  are  contemporaneous  in  origin  with  the  wall  rock.  The 
word  means  of  the  same  origin.  Trans.  Amer.  Inst.  Min.  Eng.,  XXIII. , 
211,1893.  Compare  xenogenite,  hysterogenite. 

Idiomorphic,  a  descriptive  term  for  those  component  minerals  of  a 
rock  that  have  their  own  crystal  faces.  Rarely  all  are  of  this  character, 
and  then  the  rock  is  called  panidiomnrphig.  Again,  some  are,  and 
others  are  not,  giving  the  hypidi^iorphic  texture.  The  phenocrysts  of 
porphyries  are  most  prone  to  be  I&OJQQTJ^  When  no  minerals  have 
their  own  crystal  faces,  as  in  most  granites,  the  rock  is  ^]ntriomj2£rjfci£,  as 
earlier  explained.  All  these  terms  were  suggested  by  Rosenbusch. 
Mass.  Gesh.,  1887,  but  Rohrbach's  automorphic  and  xenomorphic,  as  is 
stated  under  the  former,  have  a  year's  priority  and  mean  the  same  thing. 
The  words  are  of  chief  importance  in  microscopic  work. 

Ij elite,  a  granitoid  nephi^line  rock,  occurring  in  Finland  and  corre- 
sponding in  mineralogy  to  the  nephelinites.  It  contains  chiefly  nepheline 
and  pyroxene.  The  name  is  derived  from  the  lijoki  River,  Finland,  and 
was  given  by  Ramsay  and  Berghell.  Stockholm  geol.  foren.  forh. 
1891,  300. 

Inclusions,  the  term  is  applied  both  to  crystals  or  anhedra  of  one 
mineral  involved  in  another,  and  to  fragments  of  one  rock  inclosed  in 
another,  as  when  a  volcanic  flow  picks  up  portions  of  its  conduit. 

Infusorial  earth.     See  diatomaceous  earth. 

Intratelluric,  a  term  applied  to  those  processes  that  take  place  deep 
within  the  earth.  For  example,  the  large  phenocrysts  of  a  porphyry  are 
usually  of  intratelluric  crystallization.  See  p.  14. 

Intrusive,  the  contrasted  term  with  effusive,  and  applied  to  those 
rocks  that  have  crystallized  without  reaching  the  surface.  They  there- 
fore form  dikes,  laccolites  and  bathylites.  Plutonic  is,  to  a  certain  ex- 
tent, synonymous.  See  p.  13. 

Itabirite,  a  metamorphic  rock,  first  described  from  Brazil,  of  schistose 
structure  and  composed  essentially  of  quartz  grains  and  scales  of  specular 
hematite.  Some  muscovite  is  also  present.  It  is  a  close  relative  of 
itacolumite.  It  was  named  from  Itabira,  a  place  in  Brazil.  When  it 
crumbles  to  powder  it  is  called  jacotinga. 

Itacolumite,  or  flexible  sandstone,  is  a  peculiar  quartz  schist  first  de- 
scribed from  Brazil,  but  since  found  in  North  Carolina  and  elsewhere. 


GLOSSARY.  143 

It  is  composed  of  interlocking  quartz  grains,  to  which  it  owes  its  flexi- 
bility, of  muscovite,  talc  and  a  few  other  minerals,  and  has  been  regarded 
as  the  mother  rock  of  the  Brazilian  diamonds.  See  p.  107. 

J 

Jacupirangite,  a  name  derived  from  Jacupiranga,  Prov.  Sao  Paulo,. 
Brazil,  and  applied  by  O.  A.  Derby  to  a  group  of  igneous  rocks,  consisting 
sometimes  of  pure  magnetite;  again  of  magnetite  with  accessory  pyroxene; 
or  of  pyroxene  with  accessory  magnetite  ;  or  of  pyroxene  and  nepheline 
with  biotite  and  olivine  in  greater  or  less  quantity.  Amer.  Jour.  Sci., 
April,  1891,  314. 

/'"'Jasper,  red  chalcedony,  abundant  enough  on  Lake  Superior  and  else- 
where to  be  a  rock. 

Jaspilite,  a  name  originally  proposed  by  Wadsworth  for  all  the  acid 
eruptive  rocks,  whose  chemical  and  physical  condition  carries  them  above 
the  rhyolites,  but  now  used  more  or  less  loosely  around  Lake  Superior  for 
the  jasper  associated  with  the  local  iron  ores. 


Kaolin,  the  hydrated  silicate  of  alumina,  Al2O3,2SiO2,2H2O,  that  is- 
the  base  of  clays,  and  that  gives  them  plasticity.  See  p.  67. 

Kelyphite-rim,  a  name  applied  by  Schrauf  to  rims  of  pyroxene,  horn- 
blende and  spinel  that  sometimes  surround  the  garnets  of  peridotites.  It 
is  of  microscopic  application. 

Keratophyr,  a  rock  intermediate  between  porphyries  and  porphyrites, 
and  differing  from  either  in  having  as  the  principal  feldspar,  anorthoclase 
instead  of  either  orthoclase  or  the  soda  lime  feldspars.  Keratophyr  applies 
to  pre-tertiary  rocks,  whereas  pantellerite  is  used  for  the  same  aggregate 
of  more  recent  geological  date.  The  name  was  given  in  1874  by  Glim- 
bel  to  certain  Bavarian  felsitic  and  porphyritic  rocks  that  resembled  horn- 
fels,  hence  the  name  from  the  Greek  for  horn.  Its  significance  has  since 
been  restricted. 

Kersantite,  a  very  old  name  of  somewhat  varying  application,  but 
formerly  used  for  rocks  that  are  intermediate  between  diorites  or  their 
corresponding  porphyrites  and  gabbros  or  diabases.  Mica-diabase  was 
used  as  a  synonym.  Rosenbusch,  in  carrying  out  the  separation  of  the 
dike  rocks  from  the  effusive  and  intrusive  grand  divisions  has  sought  to 
restrict  the  name  to  those  dike  rocks  with  plagioclase  that  have  prevailing 
dark  silicates  of  which  the  chief  is  biotite.  Kersanton  is  practically  a 
synonym.  Both  names  are  derived  from  a  town  in  Brittany. 

Kies,  a  general  term  for  the  sulphide  ores,  now  adopted  into  English 
from  the  original  German. 


144  A   HANDBOOK  OF  ROCKS. 

Kieselguhr,  German  name  for  diatomaceous  earth,  and  more  or  less 
current  in  English. 

Killas,  Cornish  miner's  term  for  the  slates  or  schists  that  form  the 
country  rock  of  the  Cornish  tin  veins. 

Kimberlite,  a  name  given  by  H.  Carville  Lewis  to  the  peridotite  that 
forms  the  diamantiferous  dike  at  the  Kimberley  mines,  of  South  Africa. 
(Geol.  Magazine,  1887,  22.)  The  word  is  mis-spelled  Kimberleyite  on 
p.  5 1 .  The  rock  is  more  porphyritic  than  typical  peridotite. 

Kinzigite,  a  metamorphic  rock  consisting  of  biotite,  garnet  and  oligo- 
clase.  It  was  named  in  1860  by  Fischer,  from  the  Kinzig  Valley,  in  the 
Black  Forest.  Neues  Jahrb.,  1860,  796. 

Knotty,  a  descriptive  term  for  those  slates  or  schists  which  are  so 
altered  by  contact  metamorphism  as  to  have  new  minerals  developed  in 
them,  giving  them  a  spotted  or  knotty  appearance.  See  p.  89. 

Krablite,  ejected  blocks  from  the  volcano  of  Krafla,  in  Iceland,  which 
were  regarded  many  years  ago  by  Forchhammer,  under  the  name  baulite, 
as  a  feldspar,  of  percentage  in  silica  far  beyond  that  of  albite.  (Jour.  f. 
prakt.  Chemie,  1843,  390;  Jahresber.  iiber  die  Fortschrit.  Chemie  u. 
Mineralogie,  1844,  262.)  It  was  soon  shown  by  the  microscope  to  be  an 
aggregate. 

Kugel,  the  German  word  for  ball  or  sphere,  and  often  prefixed  to 
those  igneous  rocks  that  show  a  spheroidal  development,  such  as  corsite, 
orbicular  granite,  etc. 

Kulaite,  a  name  derived  from  the  Kula  basin  in  Lydia,  Asia  Minor, 
proposed  by  H.  S.  Washington,  for  those  rare  basalts  (there  abundant) 
in  which  hornblende  surpasses  augite  in  amount.  "  The  Volcanoes  of 
the  Kula  Basin."  Privately  printed.  New  York,  1894,  Amer.  Jour. 
ScL,  Feb.  1894,  p.  115. 


Labradorite,  the  name  of  the  feldspar  is  prefixed  to  many  rock 
*names.  Labradorite  rock  was  formerly  much  used  for  anorthosite,  which 
see. 

€-»accolite,  a  name  based  on  the  Greek  word  for  cistern  and  suggested 
by  G.  K.  Gilbert  for  those  intrusions  of  igneous  rock  that  spread  out 
laterally  between  sedimentary  beds  like  a  huge  lens,  and  that  never  reach 
the  surface  unless  exposed  by  erosion.  See  p.  1 2  ;  also  Geology  of  the 
Henry  Mountains,  Utah,  p.  19. 

Lamprophyr,  a  general  term,  now  used  in  a  somewhat  wider  sense 
than  as  originally  proposed  by  Glimbel,  who  suggested  it.  Rosenbusch, 
in  the  Massigen  Gesteine,  gave  it  its  present  significance.  Lamprophyrs 
are  dike  rocks  of  porphyritic  texture,  whose  predominant  phenocrysts  are~ 


GLOSSARY.  145 

V 

the  dark  silicates,  augite,  hornblende  or  biotite.  They  are  practically 
basic  dikes.  The  word  means  a  shining  rock,  and  was  first  applied  in 
1874  to  small  dikes  in  the  Fichtelgebirge  that  were  rich  in  biotite. 

Lapilli,  volcanic  dust  and  small  ejectments,  the  results  of  explosive 
eruptions. 

Lassenite,  Wadsworth's  name  for  unaltered  glassy  trachytes.  Kept. 
State  Geol.  Mich.,  1891-92,  p.  97.  The  name  is  derived  from  Lassen's 
Peak,  Cal. 

^  Laterite,  a  name  derived  from  the  Latin  word  for  brick  earth,  and 
applied  many  years  ago  to  the  red,  residual  soils  or  surface  products  that 
have  originated  in  situ  from  the  atmospheric  weathering  of  rocks.  They 
are  especially  characteristic  of  the  tropics.  Though  first  applied  to  altered 
basaltic  rocks  in  India,  laterite  has  had  in  later  years  a  general  applica- 
tion without  regard  to  the  character  of  the  original  rock.  Compare  sap- 
rolite.  See  pp.  116,  117. 

Laurdalite,  a  name  given  by  Brogger  to  a  coarsely  crystalline  variety 
of  nepheline-syenite,  that  is  abnormal  in  having  for  its  feldspar  natron- 
orthoclase,  rarely  natron- microcline,  instead  of  the  normal  potash  ortho- 
clase.  The  dark  silicates  are  biotite,  diallage  and  olivine.  Zeitsch.,  f. 
Kryst.,  XVI.,  28,  1890. 

Laurvikite,  a  name  applied  by  Brogger  to  a  Norwegian  variety  of 
augite-syenite  that  contains  natron-orthoclase  as  its  chief  feldspar  and  most 
abundant  mineral.  The  other  components  are  rare  plagioclase,  pyroxene, 
biotite,  barkevicite  or  arfvedsonite,  olivine  and   magnetite.    Besides  mi- 
croscopic accessories,  nepheline  is  occasionally  met.     Zeitsch.,  f.  Kryst., 
29,  1890.     Compare  pulaskite. 
,  a  general  name  for  the  molten  outpourings  of  volcanoes. 

Laxite,  Wadsworth's  name  for  the  fragmental  or  mechanical  rocks, 
especially  when  unconsolidated.  Rept.  of  State  Geol.  of  Mich.,  1891-92, 
p.  98. 

Leopardite,  a  siliceous  rock  from  North  Carolina,  spotted  with  stains 
of  manganese  oxide.  It  is  usually  considered  to  be  a  quartz-porphyry. 

Leopard  rock,  a  local  name  in  Canada,  applied  to  pegmatitic  rocks 
which  are  associated  with  the  apatite  veins  of  Ontario  and  Quebec.  See 
C.  H.  Gordon,  Bulletin  Geolog.  Society  of  America,  VII.,  122. 

Leptinite  or  Leptynite,  the  French  synonym  of  granulite  as  used 
among  the  Germans.     See  granulite. 
>£-     Leucite-basalt,  basaltic  rocks  with  olivine  in  which  leucite  replaces 

plagioclase.     See  p.  44. 

^—  Leucite-basanite,  basaltic  rocks  that  contain  both  leucite  and  pla- 
gioclase. As  contrasted  with  leucite-tephrites,  they  contain  olivine.  See 
p.  44. 

£^  Leucitite,  basaltic  rocks  without  olivine  in  which  leucite  replaces 
plagioclase.  Compare  leucite-basalt. 


146  A   HANDBOOK  OF  ROCKS. 

Leucitophyre,  a  name  formerly  used  as  a  general  one  for  the  leucite 
rocks,  but  now  by  common  consent  restricted  to  those  phonolites  that 
contain  both  leucite  and  nepheline. 

Leucite-tephrite,  basaltic  rocks  without  olivine,  that  contain  both 
plagioclase  and  leucite.  Compare  leucite-basanite. 

Leucophyre,  originally  applied  by  Glimbel  jn  1874  to  light-colored 
diabases  whose  feldspar  was  altered  to  saussurite  and  whose  augite  had 
largely  changed  to  chlorite.  Rosenbusch  restricts  it  to  diabases  poor  in 
plagioclase.  The  name  means  a  light-colored  or  white  porphyritic  rock, 

d  has  little  claim  to  consideration  either  in  etymology  or  application. 

Lherzolite,  a  variety  of  peridotite,  first  discovered  in  the  Pyrenees,, 
and  containing  olivine,  diopside  and  an  orthorhombic  pyroxene.  Much 
picotite  is  also  present.  It  is  named  from  Lake  Lherz,  by  de  la  Metherie,. 
Theorie  de  la  Terre,  II.,  281. 

Liebenerite-porphyry,  nepheline-porphyry  whose  nepheline  pheno- 
crysts  are  altered  to  muscovite.  Its  original  locality  is  near  Predazzo, 
in  the  Tyrol.  Compare  gieseckite-porphyry. 

L^  Limburgite,  porphyritic  basaltic  rocks  consisting  of  olivine  and  augite 
in  a  glassy  groundmass.  They  lack  feldspars.  See  p.  45.  The  name  is- 
derived  from  Limburg,  a  locality  on  the  Kaiserstuhl,  a  basaltic  mountain 
in  Baden.  It  was  suggested  by  Rosenbusch  in  1872,  and  at  the  same 
time  Boricky  described  similar  rocks  from  Bohemia  as  magma-basalt. 

Limestone,  the  general  name  for  rocks  composed  essentially  of  cal- 
cium carbonate.  See  p.  71. 

Lindoite,  Brogger's  name  for  certain  dike  rocks,  in  the  region  of 
Kristiania.  They  have  trachytic  texture ;  are  seldom  and  then  but 
slightly  porphyritic  ;  are  medium  to  coarsely  crystalline  in  the  larger  dikes ; 
possess  light  colors ;  and  often  lack  dark  colored  minerals.  When  such 
are  recognizable  they  are  pyrite  and  chlorite.  Ferriferous  carbonates  are 
present.  Traces  of  segirine  and  of  a  dark  alkali  hornblende  may  be  oc- 
casionally detected.  (Die  Eruptivgesteine  des  Kristianiagebietes,  I.,  13 \r 
1894.) 

Liparite,  a  synonym  of  rhyolite,  and  largely  used  for  the  latter 
among  Europeans,  though  rhyolite  is  chiefly  current  in  America  and 
England.  The  name  is  derived  from  the  Lipari  Islands,  off  the  coast  of 
Italy,  where  the  rocks  are  abundant.  It  was  proposed  by  Justus  Roth  in 
1 86 1.  Gesteins-analysen,  p.  xxxiv. 

Litchfieldite,  a  name  proposed  by  W.  S.  Bayley  for  the  variety  of 
nepheline-syenite  occurring  in  loose  boulders  near  Litchfield,  Me.,  whose 
chief  feldspar  is  albite  and  which  differs  therefore  from  normal  nephe- 
line-syenite. Bull.  Geol.  Soc.  Amer.,  III.,  243. 

Lithical,  a  term  proposed  by  L.  Fletcher  for  the  finer  textural  char- 
acters of  rocks,  /.  e. ,  those  for  which  texture  as  distinguished  from  struc- 


GLOSSARY.  147 

ture  is  employed  above.  See  p.  13.  Lithical  from  the  Greek  for  stone 
is  contrasted  with  petrical  from  the  Greek  for  rock.  Introduction  to 
the  Study  of  Rocks;  British  Museum  Handbooks,  1895. 

Lithionite-granite,  a  name  proposed  by  Rosenbusch  for  granites  with 
lithia  mica  or  lithionite. 

Lithographic  limestone,  an  exceptionally  homogeneous  and  fine- 
grained limestone,  suitable  for  lithography. 

Lithoidal,  a  descriptive  term  applied  to  those  groundmasses,  espe- 
cially of  rhyolites,  that  are  excessively  finely  crystalline,  like  porcelain, 
as  distinguished  from  glassy  varieties.  The  English  equivalent,  stony,  is 
also  used. 

^^  Lithophysse,  literally  "stone  bubbles,"  a  name  applied  to  those 
cellular  cavities  in  acidic  lavas,  obsidian,  rhyolite,  etc.,  that  have  concen- 
tric walls  and  that  are  caused  by  a  special  development  of  mineralizers  at 
that  particular  point.  They  are  usually  hemispherical  in  shape  and  on 
the  walls  may  have  various  well  crystallized  minerals.  See  p.  22. 
^  Lithosphere,  the  outer  stony  shell  of  the  earth.  See  barysphere. 

Local  metamorphism,  i.  <?.,  contact  metamorphism.     See  p.  85. 

Loess,  fine  surface  soils  chiefly  formed  of  wind-blown  dust.  See  p. 
65.  The  name  is  a  German  word,  akin  to  loose,  and  appears  to  have 
been  first  applied  geologically  in  the  Rhine  valley. 

Luciite,  Chelius'  name  from  the  Luciberg  in  Hesse,  for  finely  crystal- 
line diorite  dikes  whose  minerals  are  xenomorphic.  Notizblatt  Verein. 
f.  Erdkunde,  Darmstadt,  1892,  i. 

Luijaurite,  a  name  proposed  by  Brogger  for  a  nepheline  syenite,  rich 
in  segirine  and  eudialyte.  Zeitsch.  f.  Kryst.,  XVI.,  204.  The  name  is 
from  a  Lapland  locality,  where  the  rock  was  discovered  by  Ramsay. 

Lustre-mottlings,  a  name  applied  by  Pumpelly  to  certain  augitic 
rocks  which  have  a  shimmering  lustre  because  the  shining  cleavage  faces 
of  the  augite  crystals  are  mottled  by  small  inclusions.  Proc.  Amer. 
Acad.,  XIII.,  260,  1878.  Compare  poicilitic  and  schiller. 

Luxullianite,  a  tourmaline  granite  from  Luxullian,  in  Cornwall, 
that  is  a  product  of  contact  metamorphism.  See  p.  32. 

Lydite.     See  basanite. 


M 

Macroscopic,  a  word  formerly  current  as  a  synonym  of  megascopic, 
/'.  <?.,  recognizable  by  the  naked  eye.  It  is  etymologically  less  correct  as 
an  antithesis  of  microscopic  than  is  megascopic,  for  "  macro"  is  from 
the  Greek  for  broad,  whereas  "mega"  means  large.  Nevertheless,  it 
preceded  megascopic  in  general  use  and  is  still  current. 

Magma  is  now  generally  employed  for  the  molten  masses  of  igneous 


148  A   HANDBOOK  OF  ROCKS. 

rock  before  they  have  crystallized.  An  original  parent  magma  may 
break  up  into  several  derived  ones.  See  pp.  2,  13,  57.  The  word  is  also 
used  in  the  sense  of  basis  as  earlier  defined,  but  this  use  is  unfortunate. 

Magma-basalt,  a  synonym  of  limburgite,  which  was  proposed  by 
Boricky,  in  1872,  at  about  the  same  time  that  Rosenbusch  suggested  lim- 
burgite. Some  authorities  give  the  former  the  preference. 

Magnetite.  The  name  of  the  mineral  is  prefixed  to  the  names  of 
many  recks  in  which  it  is  prominent.  It  almost  furnishes  a  rock  itself, 
at  times. 

Malchite,  a  variety  of  diorite  dikes  which  have  in  a  groundmass  of 
quartz,  feldspar  and  hornblende,  phenocrysts  of  plagioclase,  hornblende 
and  biotite.  The  name  was  given  by  A.  Osann,  and  is  derived  from 
Malchen,  another  name  for  Mt.  Melibocus,  in  Hesse. 

Malignite,  a  name  proposed  by  Lawsdn  for  a  group  of  rocks  on  the 
Maligne  river,  Rainy  Lake  district,  province  of  Ontario.  They  are  de- 
scribed as  "basic,  holocrystalline,  plutonic  rocks,  rich  in  alkalies  and 
lime."  Iron  is  present  in  moderate  amounts  almost  entirely  combined 
in  the  silicates.  Iron  and  magnesia  are  more  abundant  than  is  usual  in 
the  alkali-rich  plutonic  rocks.  The  chief  minerals  are  orthoclase,  often 
microscopically  intergrown  with  an  acid  plagioclase;  segerine-augite, 
which  may  predominate  with  but  a  moderate  admixture  of  biotite,  or 
may  be  subordinate  and  intergrown  with  preponderant  soda  amphibole, 
biotite  being  present  as  before.  There  are  three  types  of  malignites,  one 
of  which  has  much  melanite  and  another  much  nepheline.  Bull.  Dept. 
Geol.  Univ.  Calif.,  I.,  340,  1896. 

Manganolite,  Wadsworth's  name  for  rocks  composed  <  f  manganese 
minerals,  such  as  wad,  psilomelane,  etc.  Rept.  State  Geol.  Mich., 
1891-92,  p.  93. 

Marble,  in  lithology,  a  metamorphosed  and  recrystallized  limestone. 
In  the  trade  the  name  is' applied  to  any  limestone  that-will  take  a  polish. 

Marekanite,  a  rhyolitic  perlite  from  the  banks  of  the  Marekana 
River,  near  Ochotsk,  Siberia.  At  times  a  very  clear  glass,  it  is  found  in 
balls  and  as  cores  of  large  perlitic  masses  and  may  even  be  under  strain 
like  Prince  Rupert's  drops.  See  Zirkel's  Petrographie,  II.,  299. 

Marl,  a  calcareous  clay,  or  intimate  mixture  of  clay  and  particles  of 
calcite  or  dolomite,  usually  fragments  of  shells.  Marl  in  America  is 
chiefly  applied  to  incoherent  sands,  but  abroad  compact  impure  lime- 
stones are  also  called  marls. 

Marmarosis,  the  general  name  for  the  process  of  crystallization  of 
limestone  to  marble,  whether  by  contact  or  regional  metamorphism.  It 
was  coined  by  Geikie  from  the  Latin  for  marble. 

Massive,  the  antithesis  of  stratified  and  therefore  often  used  as  a  syn- 
onym of  igneous  or  eruptive  rocks  as  contrasted  with  the  sedimentary  and 
laminated  metamorphic. 


GLOSSARY.  149 

'"**'  Megascopic,  a  descriptive  term  meaning  large  enough  to  be  dis- 
tinguished with  the  naked  eye;  the  antithesis  of  microscopic.  See 
macroscopic.  Used  also  to  describe  methods  of  observation  without  the 
microscope  or  with  the  eye  alone. 

/^  Melaphyre,  a  rock  name  first  introduced  by  Brogniart  in  1813,  practi- 
cally for  porphyritic  rocks  with  a  dark  groundmassand  feldspar  pheno- 
crysts.  After  having  had  various  meanings  for  many  years,  by  common 
consent,  it  is  now  generally  used  as  suggested  by  Rosenbusch  for  pre- 
tertiary  oli vine-basalts,  that  is,  for  porphyritic  equivalents  of  olivine-diabase. 

Melilite-basalt,  a  rare  basaltic  rock  whose  feldspathoid  is  melilite. 
It  was  first  identified  by  Stelzner  in  1882.  The  rock  is  excessively  basic. 
See  p.  45.  Alnoite  is  the  same  rock  in  dikes. 

Mesostasis,  a  synonym  of  basis  suggested  by  Giimbel. 

Metabolite,  Wadsworth's  name  for  altered,  glassy  trachytes,  of  which 
lassenite  is  the  unaltered  form.  Kept.  State  Geol.  Mich.,  1891-92,  p. 

97- 

Metachemical  metamorphism,  Dana's  term  to  describe  that  vari- 
ety of  metamorphism  that  involves  a  chemical  change  in  the  rocks  af- 
fected. Amer.  Jour.  Sci.,  July,  1886,  p.  69. 

Metadiabase,  a  shortened  form  of  metamorphic  diabase,  suggested 
by  Dana  for  certain  rocks  simulating  diabase,  but  supposed  to  have  been 
produced  by  the  metamorphism  of  sediments.  Amer.  Jour.,  Sci.,  Feb., 
1876,  121.  Compare  Pseudo-diabase. 

Metadiorite,  dioritic  rocks  produced  as  just  described  under  meta- 
diabase.  Compare  Pseudo-diorite. 

,   ^    Metamorphism,  a  collective  term  for  the  processes  by  which  rocks 
undergo  alteration  of  all  sorts.     It  is  more  fully  set  forth  on  page  84. 

Metasomatic,  /'.  e.t  a  change  of  substance;  it  is  used  to  describe  the 
replacement  of  one  or  more  of  the  minerals  of  a  rock  by  others.  The 
form  of  the  originals  is  not  at  all  preserved  as  in  pseudomorphs,  nor  does 
the  chemical  composition  remain  the  same  while  the  form  alters  as  in 
paramorphs,  but  both  customarily  change.  The  term  is  especially  used 
in  connection  with  the  origin  of  ore  deposits.  The  corresponding  noun 
is  metasomatosis,  but  replacement  is  a  good  English  equivalent. 

Metaxite,  a  name  of  Hauy's  for  micaceous  sandstone. 

Mezo  or  Meso  is  sometimes  prefixed  to  the  names  of  igneous  rocks 
of  Mesozoic  age. 

Miarolitic,  a  descriptive  term  applied  to  those  granites  that  have 
small  cavities,  into  which  well-terminated  crystals  project.  See  p.  13. 

Miascite,  a  name  coined  from  Miask,  a  locality  in  the  Urals  where  a 
nepheline  syenite  occurs  whose  dark  silicate  is  biotite.  Used  also  as  a 
general  name  for  biotitic  nepheline  syenites.  See  p.  37. 

Mica-schist,  finely  laminated  metamorphic  rocks,  consisting  of 
quartz,  mica,  feldspar  and  several  minor  minerals.  See  p.  99. 


150  A    HANDBOOK  OF  ROCKS. 


Mica-peridotite,  a  name  applied  by  J.  S.  Diller  to  a  peculiar  perido- 
tite,  occurring  as  a  dike  in  Crittenden  County,  Ky.,  and  consisting  chiefly 
of  altered  olivine  and  biotite.  Amer.  Jour.  Sci.,  Oct.  1892,  288.  See 
Analysis  19,  p.  49. 

Mica-trap,  an  English  field  name  for  dark  dike  rocks  rich  in  mica. 

Micro-felsite,  a  name  used  in  microscopic  work  for  those  varieties  of 
groundmass  that  do  not  affect  polarized  light,  but  that  are  not  true  glasses 
because  they  have  a  fibrous,  a  granular  or  some  such  texture.  The  tex- 
tures are  no  doubt  the  result  of  devitrification  of  a  glassy  base  in  many 
cases. 

'  />  Micro-granite,  a  name  used  in  microscopic  work  for  those  ground- 
masses  of  quartz-  porphyries  that  consist  of  small  quartz  and  feldspar 
crystals  with  granitoid  texture,  /".  <?.,  of  about  the  same  size  and  usually 
without  crystallographic  boundaries.  See  granophyric. 

Micro-granulite,  the  French  equivalent  of  granophyric,  as  earlier  ex- 
plained. 

£/  Micro-crystalline,  granular  rocks,  whose  components  are  recogni- 
zable, but  so  small  as  to  require  the  microscope  for  their  identification. 

Microlites,  generally  used  for  microscopic,  but  still  identifiable  miner- 
als. 

Micropegmatite,  /.  e.,  microscopic  pegmatite,  a  term  applied  to 
those  groundmasses  of  quartz-porphyries  whose  microscopic  quartz  and 
feldspars  mutually  penetrate  each  other.  The  several  parts  of  the  same 
crystal,  though  isolated,  extinguish  together.  See  granophyric. 

Microperthite,  /.  e.,  microscopic  perthite,  a  term  applied  to  that 
variety  of  orthoclase  which  is  thickly  set  with  flat  spindles  of  albite.  It 
is  very  common  in  gneisses.  Compare  granophyric. 
'  Micropoikilitic,  a  structural  term  suggested  by  G.  H.  Williams  to 
describe  those  minerals  that  are  speckled  with  microscopic  inclusions  of 
other  minerals,  having  no  definite  relations  to  each  other  or  to  their  host. 
Jour,  of  Geology,  L,  176,  1893.  Poikilitic  is  often  spelled  poicilitic  or 
poecilitic. 

Millstone-grit,  an  old  English  name  for  the  conglomeratic  sandstone 
at  the  base  of  the  Carboniferous  Coal  Measures.  It  is  more  or  less  cur- 
rent in  this  country  as  a  synonym  of  the  Great,  Pottsville  or  Serai  con- 
glomerate. 

Mineralizers,  the  dissolved  vapors  in  an  igneous  magma,  such  as 
steam,  hydrofluoric  acid,  boracic  acid  and  others,  that  exert  a  powerful 
influence  in  the  development  of  some  minerals  and  textures.  See  p.  15. 
The  word  is  also  technically  used  in  some  definitions  of  ore.  Thus  it  is 
said  that  an  ore  is  a  compound  of  a  metal  and  a  mineralizer,  such  as  cop- 
per and  sulphur,  iron  and  oxygen,  etc. 

Minette,  a  variety  of  mica-syenite,  usually  dark  and  fine-grained,  oc- 
curring in  dikes.  See  p.  35,  Anal.  6. 


GLOSSARY.  m 

Moldauite,  a  very  pure  glass,  from  the  valley  of  the  Moldau  river, 
Bohemia  See  bouteillenstein. 

Monchiquite,  a  name  suggested  by  Hunter  and  Rosenbusch  from 
the  Monchique  Mountains,  of  Portugal,  for  basaltic  dikes  corresponding 
in  mineralogy  and  texture  to  limburgite.  They  often  accompany  nephe- 
line-syenite.  Tsch.  Mitt.,  XL,  445,  1890. 

Monzonite  has  usually  been  considered  as  a  variety  of  augite-syenite 
that  displayed,  however,  considerable  mineralogical  variety.  Brogger 
has  recently  used  the  name  for  a  transitional  and  intermediate  group  of 
granitoid  rocks  between  the  granite-syenite  series  (/.  <?.,  the  alkali-feld- 
spar series)  and  the  diorites  (/.  e.,  the  lime-soda  feldspar  series).  The 
monzonites  have  both  alkali-feldspar  (or  orthoclase)  and  lime-soda  feld- 
spar (or  plagioclase)  in  approximately  equal  amounts,  or  at  least  both 
richly.  (Die  Eruptivgesteine  des  Kristianiagebietes,  II.,  21,  1895.) 

Mortar-structure,  a  term  suggested  by  Tornebohm  to  describe  those 
granites,  gneisses  or  'other  rocks  that  have  been  dynamically  crushed,  so 
that  larger  nuclei  of  their  original  minerals  are  set  in  crushed  and  com- 
minuted borders  of  the  same,  like  stones  in  a  wall. 

Mulatto,  a  local  name  in  Ireland  for  a  cretaceous  green  sand. 

Muscovado,  the  Spanish  word  for  brown  sugar,  used  by  Minnesota 
geologists  for  the  rusty,  brown,  outcropping  rock  that  resembles  brown 
sugar.  It  has  been  applied  to  both  gabbros  and  quartzites.  XVI.,  Ann. 
Rept,  Minn.  Geol.  Surv. 

Mylonite,  a  name  suggested  by  the  English,  geologist  Lapworth  for 
schists  produced  by  dynamic  metamorphism.  Rept.  of  Brit.  Assoc.,  1885- 
36,  p.  1025. 

N 

Nadel-diorite,  /.  e.,  needle-diorite,  a  German  term  for  diorites  with 
acicular  hornblende. 

Napoleonite,  a  synonym  of  corsite. 

Natron-granite,  granites  abnormally  high  in  soda,  presumably  from 
the  presence  of  an  orthoclase  rich  in  soda,  or  of  anorthoclase.  They  are 
also  called  soda-granites.  Natron  is  also  used  as  a  prefix  to  minerals  and 
rocks  that  are  rich  in  soda,  as  natron-orthoclase,  natron-syenite,  etc. 

ite,  Rosenbusch's  name  for  pre-tertiary  porphyritic  rocks,  con- 
sisting of  plagioclase,  augite  and  olivine  as  phenocrysts,  with  a  second 
generation  of  the  same  forming  the  holocrystalline  groundmass.  The 
name  is  from  Nava,  a  locality  in  the  Nahe  Valley,  Mass.  Gest.,  1887. 

Necks.  Lava-filled  conduits  of  extinct  volcanoes,  exposed  by  erosion. 

Nepheline-basalt,  an  old  general  name  for  basaltic  rocks  with  ne- 
pheline,  but  now  restricted  to  those  that  practically  lack  plagioclase,  and 
that  have  nepheline,  augite,  olivine  and  basis.  See  p.  44. 


A   HANDBOOK  OF  ROCKS. 

Nepheline-basanite,  basaltic  rocks  with  plagioclase,  nepheline,  au- 
gite, olivine  and  basis.  Compare  nepheline  t^hrite.  See  p.  44. 

Nephelinite,  basaltic  rocks  consisting  of  nepheline,  augite  and  basis, 
but  without  olivine.  See  p.  .44. 

Nephelinitoid,  Boricky's  term  now  used  in  microscopic  work  for 
nepheline-glass,  or  the  glassy  basis  in  nepheline  rocks,  whose  easy  gela- 
tinization  indicates  its  close  relations  with  this  mineral ;  unindividualized 
nepheline. 

Nepheline-syenite,  /.  e.,  eleolite-svenite,  a  name  to  be  preferred  to 
the  latter  as  there  is  no  real  need  of  the  word  eleolite.  Granitoid  rocks 
consisting  of  orthoclase,  nepheline,  and  one  or  more  of  the  following : 
hornblende,  augite  and  biotite.  The  rocks  result  from  magmas  espe- 
cially rich  in  alkalies,  and  possess  great  scientific  interest  on  account  of 
their  richness  in  rare,  associated  minerals.  See  p.  36. 

Nephelite,  a  later  method  of  spelling  nepheline  and  one  consistent 
with  approved  mineralogical  orthography. 

Nevadite,  a  name  coined  by  von  Richthofen  from  Nevada,  for  those 
rhyolites  that  approximate  a  granitoid  texture,  *'.  ^.,with  little  ground- 
mass.  Mem.  Calif.  Acad.  Sci.,  I.,  p.  54,  1867.  See  p.  24  and  Hague 
and  Iddings,  Amer.  Jour.  Sci.,  June,  1884,  461. 

Neve,  a  French  synonym  of  firn. 

Nonesite,  porphyrites  with  orthorhombic  pyroxene.  The  name  was 
given  by  Lepsius.  Das  westliche  Siid-Tyrol,  Berlin,  1878. 

Nordmarkite,  Brogger's  name  for  a  variety  of  granitic  rocks  consist- 
ing of  orthoclase,  some  oligoclase,  more  or  less  microperthite,  quartz  and 
somewhat  subordinate  biotite,  pyroxene,  hornblende  and  aegirine.  It  is 
chemically  high  in  silica  and  the  alkalies.  Zeitsch.  f.  Kryst.,  XVI.,  54, 
1890. 

Norite,  rocks  of  the  gabbro  family  that  consist  of  plagioclase  and  or- 
thorhombic pyroxene,  usually  hypersthene.  The  name  has  had  a  variable 
history  and  was  originally  proposed  in  1832  by  Esmark  for  aggregates  of 
feldspar  and  hornblende  which  were  lacking  or  notably  poor  in  diallage 
and  hypersthene.  But  as  many  localities  were  cited  in  later  years  which 
on  microscopic  examination  were  found  rich  in  these  minerals,  Rosen- 
busch  finally  gave  the  name  its  above  definition  and  this  is  its  gener- 
ally accepted  signification. 

Normal  metamorphism,  /.  <?.,  regional  metamorphism.  See  p. 
402. 

/^-  Normal-pyroxenic,  Bunsen's  name  for  his  assumed  typical  basic  ig- 
neous magma  with  48  per  cent.  SiO2  as  contrasted  with  the  correspond- 
ing normal-trachytic  one  with  76  per  cent.  SiO2.  He  sought  to  explain 
all  intermediate  rocks  by  the  intermingling  of  these  two.  Although  ap- 
parently applicable  at  times  and  serviceable  in  their  day,  the  conceptions 
have  long  since  been  exploded.  See  J.  Roth's  Gesteinsanalysen,  1861. 


GLOSSARY.  153 

Nosean.  The  name  of  the  mineral  is  often  prefixed  to  the  names  of 
rocks  containing  it. 

Novaculite,  excessively 'fine  grained  quartzose  rocks  supposed  to  be 
of  sedimentary  origin  from  siliceous  slimes.  They  are  especially  devel- 
oped in  Arkansas,  and  are  much  used  as  whetstones.  See  p.  64. 


o 

r  Obsidian,  a  general  name  for  volcanic  glass.  When  used  alone  it 
implies  a  rhyolite-glass,  but  it  is  now  much  employed  with  a  prefix  as 
andesite-obsidian,  basalt-obsidian.  See  p.  21. 

Ocellar-structure,  a  microscopic  term  used  by  Rosenbusch  for  pe- 
culiar aggregates  of  small  pyroxenes  that  resemble  eyes,  buds  and  the 
like,  and  that  are  especially  common  in  nepheline  and  leucite  rocks. 
Mass.  Gest,  625,  1887. 

Odinite,  a  name  given  by  Chelius  to  certain  porphyritic  dikes  in  Mt. 
Melibocus  which  have  a  groundmass  of  plagioclase  and  hornblende  rods, 
with  phenocrysts  of  plagioclase  and  augite  Notizbl.  Ver.  Erdkunde, 
Darmstadt,  1892,  Heft  13,  p.  i. 

Olivine.  The  name  of  the  mineral  is  prefixed  to  the  names  of  many 
rocks  that  contain  it.  It  is  of  especial  importance  in  this  respect,  as  its 
presence  marks  a  more  basic  development  in  many  rocks  as  contrasted 
with  their  varieties  that  lack  it. 

Oolitic,  a  textural  term  for  those  rocks  that  consist  of  small  concre- 
tions, analogous  to  the  roe  of  fish.  Oolites  are  calcareous,  siliceous  and 
ferruginous. 

Opacite,  a  noncommittal  microscopic  term,  less  current  than  formerly, 
for  minute  opaque  grains  observed  in  thin  sections  of  rocks.  They  are 
generally  regarded  to-day  as  chiefly  magnetite  dust. 

Ophicalcite,  Brogniart's  name  for  crystalline  limestones,  spotted 
with  serpentine.  Seep.  113. 

Ophiolite,  Brogniart's  name  for  the  serpentines.  Seep.  113.  It  is 
also  employed  in  America  in  the  sense  of  ophicalcite  as  above  given. 

Ophite,  a  name  given  in  1798  by  the  Abbe  Palassou  to  a  green  rock 
of  the  Pyrenees.  It  was  later  recognized  to  be  composed  of  feldspar  and 
hornblende,  and  still  later  was  determined  by  Zirkel  to  be  a  uralitized 
diabase.  The  name  has  chief  significance  to-day  b^ause^sea  to  de- 
scribe the  textural  peculiarity  of  diabase.  An  ophite-texture  is  one  in 
which  rod-like  or  lath-shaped,  automorphic  plagioclase  feldspars  form  an 
interlaced  aggregate,  in  whose  interstices  are  xenomorphic  augites  and 
magnetites.  The  significance  of  it  is  that  the  feldspars  crystallized  be- 
fore the  augite,  contrary  to  the  usual  succession.  See  p.  44  and  p.  17. 

Orbicular,  a  textural  term  for  those  rare  rocks  whose  minerals  have  a 

STIVERS: 


154  A   HANDBOOK  OF  ROCKS. 

spheroidal  grouping,  such  as  corsite  and  orbicular  granite.  See  kugel 
and  spheroidal. 

Orbite,  a  name  proposed  by  Chelius  for  certain  diorite  dikes  near 
Orbeshohe,  Hesse,  of  porphyritic  texture  and  having  large  phenocrysts  of 
hornblende,  biotite  and  plagioclase.  Notizbl.  Ver.  Erd.  Darmstadt, 
1892,  i. 

Orthoclase.  The  name  of  the  mineral  is  often  prefixed  to  the  names 
•of  rocks  that  contain  it. 

Orthophyre,  /.  e.,  orthoclase  porphyry  or  porphyry  proper. 

Ortlerite,  a  name  given  by  the  Austrian  geologists,  Stache  and  von 
John,  to  certain  porphyrites  of  the  eastern  Alps  that  resemble  the  old 
greenstones  and  that  have  plagioclase,  hornblende,  generally  augite,  and 
more  or  less  basis.  They  range  from  48-54  SiO2.  Jahrb.  k.  k.  g., 
Reichsanst,  1879,  342. 

Ottrelite  schists,  schistose  rocks  with  the  peculiar  micaceous  mineral 
ottrelite.  They  are  best  known  from  the  Ardennes,  Belgium,  but  are 
found  in  New  England. 

Ouachitite  (pronounced  waw-shee-tite),  a  name  coined  by  Kemp 
from  the  Ouachita  River,  Arkansas,  for  certain  basic  dikes  containing,  in 
a  glassy  groundmass,  prevailing  and  often  phenomenally  large  phenocrysts 
of  biotite,  very  subordinate  augite  and  magnetite.  They  also  occur  at 
Beemerville,  N.  J.,  associated  with  nepheline  syenite.  Ann.  Rep.  Geol. 
Surv.  of  Ark.,  1890,  II.,  393. 


Paisanite,  a  name  proposed  by  Osann  from  the  Paisano  Pass,  on  the 
Southern  Pacific  R.  R.,  in  western  Texas,  for  a  variety  of  quartz-por- 
phyry, consisting  of  microperthitic  orthoclase  and  rarer  quartz  pheno- 
crysts, in  a  groundmass  of  quartz  and  feldspar.  Occasional  groups  of 
small  hornblendes  (riebeckite)  are  met.  Tscherm.  Min.  u  Petr.  Mitth., 
XV.,  435. 

Palaeophyre,  Gumbel's  name  given  in  1874  to  certain  porphyritic 
dike  rocks  corresponding  to  quartz-mica-diorites  in  mineralogy.  They 
cut  the  silurian  strata  of  the  Fichtelgebirge. 

Palseophyrite,  a  name  proposed  by  Stache  and  von  John  (compare 
ortlerite)  for  certain  porphyrites  in  whose  strongly  prevailing  groundmass 
are  phenocrysts  of  plagioclase,  hornblende  and  augite.  Jahrb.  d.  k.  k.  g. 
Reichsanstalt,  1879,  342. 

Palseopicrite,  a  name  proposed  by  Giimbel  in  1874,  in  his  paper, 
11  Die  palaeolithischen  Eruptiv-gesteine  des  Fichtelgebirges, "  a  contribu- 
tion to  which  we  are  indebted  for  a  great  number  of  useless  and  unneces- 
sary rock  names,  for  picrites  regarded  by  him  similar  to  those  originally 


GLOSSARY.  155 

named  by  Tschermak  from  cretaceous  strata,  but  which  were  called 
palaeopicrites  because  occurring  in  palaeozoic  strata.  He  regarded  them 
as  aggregates  of  olivine,  enstatite,  chrome-diopside  and  magnetite,  but 
they  are  now  known  to  be  chiefly  olivine  and  augite.  More  or  less  brown 
hornblende  and  biotite  also  occur. 

Palagonite-tuff,  an  altered  basaltic  tuff  from  Palagonia,  in  Sicily. 
The  name  palagonite  was  originally  applied  to  problematical,  brown  in- 
clusions in  the  tuff  which  were  thought  at  first  to  be  a  definite  mineral. 
They  are  now  known  to  be  a  devitrified  basaltic  glass.  The  name  was 
given  by  v.  Waltershausen  in  1846.  See  Vulk.  Gesteine  in  Sicilien  und 
Island,  1853,  179. 

Palatinite,  a  name  proposed  by  Laspeyres  for  certain  rocks  in  the 
-German  province  of  Pfalz,  supposed  by  him  to  be  gabbros  with  diallage 
-and  of  carboniferous  age;  but  they  have  since  been  shown  to  be  essen- 
tially diabases.  Neues  Jahrb,  1869,  516.  The  word  is  derived  from 
the  classic  name  of  the  district. 

Pallasite,  originally  proposed  by  Gustav  Rose  for  a  meteorite  that 
fell  near  Pallas,  in  Russia,  it  has  been  used  by  Wadsworth  in  a  wider 
sense  for  both  meteoric  and  terrestrial,  ultra  basic  rocks,  which  in  the 
former  average  about  60%  iron,  and  in  the  latter  have  at  least  more  iron 
oxides  than  silica.  Cumberlandite  (which  see)  is  the  chief  example. 
Lithological  Studies,  1884,  68. 

'  Panidiomorphic,  Rosenbusch's  term  for  those  rocks,  all  of  whose 
components  possess  their  own  crystal  boundaries. 

Pantellerite,  a  group  ot  rocks  intermediate  between  the  rhyolites  and 
trachytes  on  the  one  hand  and  the  dacites  on  the  other.  They  differ 
from  all  these  in  having  anorthoclase  as  the  principal  feldspar.  Cossyrite, 
a  rare  and  probably  titaniferous  amphibole,  occurs  in  the  original  locality 
on  the  island  of  Pantelleria,  in  the  Mediterranean.  See  p.  27.  The 
-name  was  given  by  Foerstner.  Zeitschr.  f.  Kryst.,  1881,  348. 

Paragenesis,  a  general  term  for  the  order  of  formation  of  associated 
minerals  in  time  succession,  one  after  the  other.  To  study  the  paragenesis 
is  to  trace  out  in  a  rock  or  vein  the  succession  in  which  the  minerals  have 
developed. 

Paramorphism,  the  passage  of  one  mineral  into  another  without 
change  of  composition,  as  augite  into  hornblende  in  uralitization.  It  is 
also  used  in  connection  with  metamorphism  to  describe  such  thorough 
changes  in  a  rock  that  its  old  components  are  destroyed  and  new  ones 
are  built  up. 

Parophite,  a  name  given  by  T.  Sterry  Hunt,  Geol.  Surv.  Can.,  1852, 
•95,  to  a  rock  or  mineral  similar  to  dysyntribite.  The  name  means  like 
serpentine. 

Pearl-diabase,  see  variolite. 


«    /^ 


156  A   HANDB19K  *F  R9CKS. 

Pearlite  or  Pearlstone,  volcanic  glass  with  concentric,  shelly  tex- 
ture and  usually  with  a  notable  percentage  of  water.  See  p.  21. 

Pegmatite,  originally  applied  to  graphic  granite,  but  of  later  years 
used  as  a  general  name  for  very  coarse  dike  or  vein  granites,  such  as  have 
large  quartz,  feldspar,  muscovite,  biotite,  tourmaline,  beryl  and  other 
characteristic  minerals,  and  are  often  called  giant-granite.  See  p.  31. 
Pele's  Hair,  a  fibrous  basaltic  glass  from  the  Sandwich  Islands, 
named  after  a  local  heathen  goddess. 

Pelite,  a  general  name  for  mud  rocks,  /.  <?.,  shales,  clays  and  the  like. 
Pencatite,  see  predazzite. 

r  Peridotite,  granitoid  rocks  consisting  of  olivine  and  pyroxene  with  no 
feldspar.  See  p.  51.  Many  varieties  have  been  made  depending  on  the 
kind  of  pyroxene  present,  or  on  its  absence  in  favor  of  related  minerals, 
viz  : 

Olivine,  augite  —  Picrite. 

Olivine,  diopside  (diallage),  enstatite  —  Lherzolite. 

Olivine,  enstatite  —  Saxonite,  harzburgite. 

Olivine,  enstatite,  augite  —  Buchnerite. 

Olivine,  augite,  garnet  —  Eulysite  (metamorphic  1). 

Olivine,  diallage,  hornblende  —  Wehrlite. 

Olivine,  hornblende  —  Cortlandtite. 

Olivine,  biotite  —  Mica-peridotite. 

Olivine,  hornblende  (secondary?),  biotite  —  Scyelite. 

Olivine,  alone  or  with  chromite  —  dunite. 

Further  particulars  about  each  of  these  will  be  found  under  the  individ- 
ual names.     Compare  also  kimberlite. 

Perthite,  a  name  given  by  T.  Sterry  Hunt,  to  parallel  intergrowths  of 
orthoclase  and  albite,  originally  described  from  Perth,  Ontario. 

Petrical.  L.  Fletcher's  name  for  the  coarser  structural  features  of 
rocks.  See  lithical. 

Petrography,  properly  the  descriptive  part  of  the  science  of  rocks 
for  which  the  more  general  name  is  petrology  or  lithology,  but  petrog- 
raphy is  widely  used  as  a  synonym  of  the  latter. 

Petrosilex,  an  old  name  for  extremely  finely  crystalline  porphyries 
and  quartz-porphyries  and  for  those  finely  crystalline  aggregates  we  now 
know  to  be  devitrified  glasses  ;  also  for  the  ground  masses  of  the  former 
which  though  not  glassy  are  yet  not  resolvable  by  the  microscope  into 
definite  minerals.  See  felsite,  micro-felsite.  It  was  practically  a  confes- 
sion by  the  older  petrographers  that  they  did  not  know  what  the  rock 
consisted  of. 

Phenocrysts,  a  name  suggested  by  J.  P.  Iddings  (Bull.  Phil.  Soc. 
Wash.,  XI.,  73,  1889),  for  porphyritic  crystals  in  rocks.  It  has  proved 
an  extremely  convenient  one,  although  its  etymology  has  been  criticised. 
It  may  be  best  to  change  to  phanerocryst,  just  as  in  botany  phenogam  has 


GLOSSARY.  157 

yielded  to  phanerogam  ;  but  one  form  or  the  other  is  an  absolute  neces- 
sity. 

Phonolite,  volcanic  rocks,  of  porphyritic  or  felsitic  texture,  consist- 
ing of  orthoclase,  nepheline,  pyroxene  and  more  rarely  amphibole.  Leu- 
cite  may  replace  the  nepheline  yielding  leucite-phonolites.  See  p.  28. 
The  name  is  Klaproth's  rendering  into  Greek  of  the  old  name  clinkstone. 
Abhandl.  Berlin  Akad.,  1801. 

Phosphorite,  massive  calcic  phosphate,  of  the  composition  of  apatite 
but  usually  lacking  crystal  form. 

Phosphorolite,  Wadsworth's  name  for  phosphatic  rocks,  guano-phos- 
phorite, apatite,  etc.  Rept.  State  Geol.  Mich.,  1891-92,  p.  93. 

Phthanite,  Hauy's  name  for  silicious  schists.  Its  use  has  recently 
been  revived  in  America  by  G.  F.  Becker,  who  applies  it  to  certain 
silicified  shales  in  California.  Quicksilver  Deposits  of  the.  Pacific  Coast, 
Mono.,  XIII.,  105,  U.  S.  Geol.  Survey. 

Phyllite,  intermediate  rocks  between  the  mica-schists  and  slates, 
usually  finely  crystalline ;  mica-slates.  Seep.  101. 

Phyre.  The  last  syllable  of  porphyry,  often  used  with  other  prefixes, 
as  vitrophyre,  orthophyre,  granophyre,  etc. 

Picrite,  a  name  originally  given  by  Tschermak  to  certain  porphyritic 
rocks  from  the  Carpathians,  that  have  abundant  and  large  phenocrysts  of 
olivine,  with  less  pyroxene,  hornblende  and  biotite,  in  a  glassy  ground- 
mass,  more  or  less  devitrified.  The  rocks  are  practically  pretertiary  lim- 
burgites.  Picrite  is  now  also  applied  to  those  peridotites  that  consist  of 
olivine  and  augite.  It  is  derived  from  the  Greek  for  bitter  in  allusion  to 
the  high  percentage  of  magnesia,  Bittererde  in  German. 

Pistazite,  a  synonym  of  epidote,  more  current  in  Europe  than  Amer- 
ica, and  used  in  rock  names  for  epidote. 

Pitchstone,  a  glassy  rock,  usually  corresponding  to  the  rhyolites  or 
trachytes,  but  with  a  considerable  percentage  of  water,  5-8  %  for  example. 
It  was  formerly  specially  used  for  pretertiary  glasses,  /.  e.,  the  glasses  of 
quartz-porphyries  and  porphyries,  but  time  distinctions  are  obsolete. 
Pitchstones  have  a  marked  resinous  luster  as  the  name  implies.  See  p.  21. 

Plutonic,  a  general   name  for  those  rocks  that  have  crystallized  in ; 
depth,  and  have  therefore  assumed  as  a  rule,  the  granitoid  texture.    See 

P-  13- 

Pneumatolitic,  a  general  name  for  those  minerals  which  have  been 
produced  in  connection  with  igneous  rocks  through  the  agency  of  the 
gases  or  vapors  called  mineralizers.  They  may  be  in  the  igneous  mass 
itself  or  in  cracks  in  the  wall  rock.  Compare  the  cases  cited  on  p'p. 
91-92.  The  term  is  much  used  in  discussions  of  ore  deposits. 

Poicilitic,  /.  e.,  speckled,  a  term  proposed  by  G.  H.  Williams  for 
those  rocks  which  have  a  mottled  luster,  because  on  the  shining  cleavage 


158  A   HANDBOOK  OF  ROCKS. 

faces  of  some  of  their  minerals,  small  inclusions  of  others  occur,  produc- 
ing the  effect.  The  same  thing  was  earlier  called  "luster-mottling"  by 
Pumpelly,  but  poicilitic  has  proved  a  useful  term  both  in  megascopic  and 
microsopic  work.  (Journal  of  Geology,  I.,  176,  1893.)  It  is  also 
spelled  poikilitic. 

Porcellanite,  fused  shales  and  clay,  that  occur  in  the  roof  and  floor 
of  burned  coal  seams.  The  rock  is  quite  common  in  the  lignite  districts 
of  the  West,  where  apparently  spontaneous  combustion  has  fired  the 
seams  in  the  past. 

Porodine,  Breithaupt's  name  for  amorphous  rocks,  such  as  are  de- 
rived from  gelatinous  silica. 

Porodite,  Wadsworth's  name  proposed  in  1879,  for  all  the  altered 
fragmental  forms  of  eruptive  rocks,  commonly  called  diabase  tuff,  schal- 
stein,  etc.  Bull.  Mus.  Comp.  Zool.,  1879,  V.,  280. 

Z^  Porphyrite,  a  porphyritic  rock,  belonging  to  the  plagioclase  series  and 
corresponding  in  mineralogy  to  the  diorites.  To  distinguish  it  from 
andesite  it  is  necessary  to  draw  a  contrast  between  surface  flows  (ande- 
sites)  and  intruded  dikes  or  sheets,  (porphyrites)  ;  or  between  tertiary  and 
later  lavas  (andesites)  and  pre-tertiary  ones  (porphyrites);  or  between 
those  with  glassy  or  very  finely  crystalline  groundmasses  (andesites)  and 
those  with  groundmasses  of  moderate  coarseness  (porphyrites). 
A'  Porphyritic,  a  textural  term  for  those  rocks  which  have  larger  crys- 
tals (phenocrysts)  set  in  a  finer  groundmass,  which  may  be  crystalline 
or  glassy,  or  both.  See  p.  13.  Rosenbusch  has  sought  to  define  it  as 
the  texture  due  to  the  recurrence  of  the  period  of  crystallization  of  the 
same  or  similar  minerals  (Neues  Jahrb.,  1882,  II.,  3).  While  except  for 
porphyritic  rocks  with  a  glassy  groundmass  this  practically  amounts  to 
the  same  thing  as  the  textural  definition  just  given,  it  is  idle  for  any  writer 
to  try  to  change  so  old,  well-established  and  indispensable  a  conception. 
jS'  Porphyry,  a  word  derived  from  the  classic  name  of  the  shell  fish,  a 
species  of  Murex,  that  yielded  the  Tyrian  purple  of  the  ancients.  It  was 
later  applied  to  the  red  porphyritic  rock  of  the  Egyptian  quarries,  "por- 
fido  rosso  antico,"  whose  red  color  is  due  to  piedmontite,  a  manganese 
epidote.  In  course  of  time  it  was  applied  to  all  porphyritic  rocks  as  we 
now  understand  the  term.  In  its  restricted  sense  it  implies  orthoclase- 
porphyry,  the  porphyritic  rock  corresponding  to  syenite,  but  to  give  it 
any  essential  significance  as  contrasted  with  trachyte,  one  of  the  three 
distinctions  must  be  drawn,  which  are  cited  above  under  porphyrites,  and 
of  which  the  second  is  of  no  real  value.  See  p.  24.  Porphyry  is  collo- 
quially used  for  almost  every  igneous  rock  in  the  West,  that  occurs  in 
sheets  or  dikes  in  connection  with  ore  bodies. 

Porphyroid,  metamorphic  rocks  with  porphyritic  texture,  /.  <?.,  with 
phenocrysts  of  feldspar  or  other  minerals  in  a  finer  groundmass,  yet  shown 


GLOSSARY. 


159 


by  geological  relations  to  be  altered  sediments,  or  tuffs.  Fossil  remains 
have  even  been  detected  in  some.  They  are  close  relatives  of  halleflintas. 

Pozzuolane,  a  leucitic  tuff,  found  near  Naples  and  used  for  hydraulic 
cement. 

Predazzite,  a  contact  rock  produced  at  Predazzo  in  the  Tyrol  by  an 
intrusion  of  syenite  in  crystalline  dolomite.  It  is  part  calcite  and  part 
brucite  or  hydromagnesite.  Pencatite  is  the  same  aggregate,  darkened 
by  grains  of  pyrrhotite. 

Primary,  an  old  synonym  of  Archean.  Also  used  for  those  rocks 
which  have  crystallized  directly  from  fusion  or  solution,  as  contrasted 
with  transported  or  secondary  sediments. 

Propylite,  a  name  given  by  von  Richthofen  in  1867  to  certain  ande- 
sites,  formed  at  the  beginning  of  tertiary  time,  that  were  thought  to  re- 
semble the  old  diorites,  and  diorite-porphyrites.  They  had  been  previ- 
ously called  by  him  greenstone-trachytes  in  Hungary,  but  were  not 
named  propylite  until  he  met  them  again  in  Nevada  and  California  (Me- 
moirs of  the  California  Academy  of  Sciences,  I.,  60,  1867).  The  west- 
ern propylites  have  been  since  conclusively  shown  by  several  American 
petrographers  to  be  only  more  or  less  altered  andesites.  The  literature 
of  the  name  furnishes  an  interesting  and  amusing  exhibition  of  the  efforts 
of  those  petrographers  who  are  influenced  by  the  time-myth  in  the  classi- 
fication of  igneous  rocks  to  draw  distinctions  where  there  are  no  differ- 
ences. The  name  means  before  the  gates,  alluding  to  their  position  at 
the  beginning  or  entrance  to  the  tertiary,  which  was  supposed  to  usher 
in  the  true  volcanic  eruptions  of  geological  time.  See  p.  41. 

Proteolite,  an  old  name  for  certain  contact  rocks  produced  by  gran- 
ite intrusions  from  slates  in  Cornwall.  It  has  been  lately  revived  by 
Bonney  for  andalusite-hornfels.  (Q.  J.  G.  S.,  1886,  104.)  Compare 
Cornubianite. 

Proterobase,  originally  applied  by  Giimbel,  1874,  to  Silurian  or  ear- 
lier diabases  with  hornblende.  The  frequency  of  the  paramorphism  of 
augite  to  hornblende  has  led  others  to  apply  it  to  diabases  with  uralitized 
augite.  Rosenbusch  restricts  it  to  diabases  with  original  hornblende. 

Protogine,  an  old  name  for  a  granite  or  gneiss  in  the  Alps,  consist- 
ing of  quartz,  orthoclase  and  chlorite  or  sericite,  which  was  formerly 
erroneously  taken  for  talc.  The  laminated  structure  from  dynamic  met- 
amorphism  is  often  pronounced. 

Psammite,  a  general  name  for  sandstones,  from  the  Greek  word  for 
a  grain  of  sand. 

Psephites,  a  general  name  for  conglomerates  and  breccias,  /.  <?., 
coarse  fragmental  rocks  as  contrasted  with  psammites  and  pelites.  The 
name  is  derived  from  the  Greek  for  pebble. 

Pseudo-diabase,  a  name  proposed  by  G.  F.  Becker  for  certain  met- 


160  A   HANDBOOK  OF  ROCKS. 

amorpnic  rocks  in  the  coast  ranges  of  California  that  have  been  derived 
from  sediments  yet  that  have  the  minerals  and  texture  of  diabase.  Mon- 
ograph, XIII. ,  U.  S.  Geol.  Surv.,  p.  94.  Compare  Metadiabase,  which 
means  the  same  thing  and  has  precedence. 

Pseudo-diorite,  dioritic  rocks  produced  as  described  under  pseudodia- 
base  above.  See  the  same  reference. 

Pseudo-chrysolite,  synonym  of  moldauite,  bouteillenstein. 

Pseudomorph,  the  replacement  of  one  mineral  by  another,  such  that 
the  form  of  the  first  is  preserved  by  the  second,  despite  the  difference  in 
composition. 

Puddingstone,  conglomerate. 

Pulaskite,  a  special  name  given  by  J.  Francis  Williams  to  certain 
syenitic  rocks  from  Pulaski  County,  Arkansas,  that  have  trachytic  texture 
and  that  consist  of  orthoclase  (kryptoperthite),  hornblende  (arfvedsonite), 
biotite  and  a  little  augite  (diopside),  eleolite,  sodalite  and  accessory  min- 
erals. Ann.  Rep.  Geol.  Surv.  Ark.,  1890,  II.,  56.  Compare  laurvikite. 

Pumice,  excessively  cellular,  glassy,  lava,  generally  of  the  composi- 
tion of  rhyolite.  See  p.  21. 

Pyroschists,  a  name  suggested  by  T.  Sterry  Hunt  for  those  sedi- 
ments that  are  impregnated  with  combustible,  bituminous  matter.  Amer. 
Jour.,  March,  1863,  159. 

Pyroxene.  The  name  of  the  mineral  is  often  prefixed  to  the  name 
of  the  rocks  that  contain  it. 

Pyroxenite,  a  name  first  proposed  by  T.  Sterry  Hunt  for  the  masses 
of  pyroxene  occurring  with  the  apatite  deposits  of  Canada.  It  is  now 
generally  employed  in  the  sense  advocated  by  G.  H.  Williams  for  grani- 
toid, non-feldspathic  rocks,  whose  chief  mineral  is  pyroxene,  and  which 
lack  olivine.  Seep.  51.  (Amer.  Geologist,  July,  1890,  p.  47.)  Wil- 
liams proposed  the  name  websterite,  from  Webster,  N.  C.,  for  a  variety 
consisting  of  diopside  and  bronzite,  with  the  latter  porphyritically  devel- 
oped. Idem.,  35. 

Q 

Quartz,  the  name  of  the  mineral  is  prefixed  fo  the  names  of  many 
rocks  that  contain  it,  as  quartz-porphyry,  p.  24 ;  quartz-trachyte,  p.  24, 
etc. 

Quartzite,  metamorphosed  sandstone.  See  p.  106.  Not  to  be  used 
for  vein  quartz. 

R 

Reaction-rims,  a  term  mostly  used  in  microscopic  work,  for  the 
curious  rims  of  hypersthene,  garnet,  hornblende,  biotite,  magnetite  and 


GLOSSARY.  161 

perhaps  other  minerals,  that  surround  grains  of  magnetite  or  of  ferro- 
magnesian  silicates  wherever,  in  many  gabbros  they  come  next  to  feJd- 
spar.  They  are  supposed  to  be  produced  by  the  reaction  of  the  minerals 
on  each  other,  probably  in  the  crystallization  of  the  rock.  (See  J.  F. 
Kemp,  Bull.  GeoL  Soc.  of  Amer.,  V.  221,  1894). 

Regional-metamorphism,  Daubree's  name  for  that  extended 
metamorphism,  that,  as  contrasted  with  contact  effects,  is  manifested  over 
large  areas  See  pp.  85-93. 

Rensselaerite,  E.  Emmons'  name  for  a  talcose  rock  from  St.  Law- 
rence Co.,  N.  Y.  Annual  Report  of  the  N.  Y.  Geol.  Survey,  1837,  p. 
152. 

Resorbed,  a  term  used  in  microscopic  work  to  describe  those  pheno- 
crysts  which  after  crystallization  are  partially  fused  again  into  the  magma. 
See  p.  17. 

Retinite,  the  current  name  for  pitchstone  among  the  French. 

Rhomben-porphyries,  a  name  applied  to  certain  Norwegian  por- 
phyries, whose  phenocrysts  of  orthoclase  are  bounded  by  oo  P  and  2Poo  , 
so  that  they  resemble  a  rhombohedron.  The  orthoclase  is  rich  in  soda. 
^Rhyolite,  volcanic  rocks,  of  porphyritic  or  felsitic  texture,  whose 
phenocrysts  are  prevailingly  orthoclase  and  quartz,  less  abundantly  bio- 
tite,  hornblende  or  pyroxene,  and  whose  groundmass  is  crystalline,  glassy, 
or  both.  The  name  is  from  the  Greek  to  flow,  and  refers  to  the  frequent 
flow  structure.  Rhyolite  is  current  in  America,  whereas  liparite  and 
quartz- trachyte  are  more  used  abroad.  The  name  was  given  in  1860  by 
v.  Richthofen.  (Jahrb.  d.  k.  k.  R.eichsanst,  XL,  153,  1860). 

Rill-marks,  small  depressions  in  sandstones,  produced  by  the  eddy- 
ing of  a  retreating  wave  on  a  seabeach  under  the  lee  of  some  small  ob- 
struction such  as  a  shell  or  pebble. 

Ripple-marks,  corrugations  in  sandstones  produced  by  the  agitation 
of  waves  or  winds  when  the  rock  was  being  deposited. 

Rock-flour,  a  general  name  for  very  finely  pulverized  rocks  or 
minerals  which  lack  kaolin  and,  therefore,  the  plasticity  of  clay,  and 
which  are  much  finer  than  sand.  Rock-flour  which  is  largely  pulverized 
quartz  may  be  separated  from  most  clays. 

s 

Saccharoidal,  a  textural  term  applied  to  sandstones  that  resemble 
old-fashioned  loaves  of  sugar. 

Sagvandite,  a  curious  rock  from  near  Lake  Sagvand,  Norway,  that  is 
mainly  bronzite  and  magnesite.  A  little  colorless  mica,  and  more  or  less 
chromite  and  pyrite  are  also  present.  The  name  was  given  by  Pettersen. 
Neues  Jahrb.,  1883,  II.,  247. 

Sahhte,  a  variety  of  pyroxene,  sometimes  prefixed  to  rock  names. 


1 62  A   HANDBOOK  OF  ROCKS. 

Salband,  a  term  current  among  miners  for  the  parts  of  a  vein  or  dike 
next  to  the  country  rock. 

Sand,  incoherent  fragment  of  minerals  or  rocks  of  moderate  size,  say 
one-quarter  of  an  inch  (6  mm. )  and  less  in  diameter.  Quartz  is  much 
the  commonest  mineral  present.  See  p.  63. 

Sandstone,  consolidated  sandf.     See  p.  63. 

Sanidinite,  a  name  applied  especially  to  certain  trachytic  bombs  that 
occur  in  tuffs  in  the  extinct  volcanic  district  of  theLaacher  See,  Germany. 
Recently  it  has  been  suggested  by  Weed  and  Pirsson  for  those  syenitic 
rocks  which  are  all  orthoclase.  Amer.  Journ.  Sci.  Dec.,  1895,  p.  479. 

Sanukite,  Weinschenk's  name  for  a  glassy  phase  of  andesite  that  con- 
tains bronzite,  augite,  magnetite,  and  a  few  large  plagioclases  and  garnets. 
They  are  related  to  the  andesites  as  are  the  limburgites  to  the  basalts. 
Neues  Jahrb.  Beilageband,  VII. ,  148,  1891. 

Saussurite-gabbro,  gabbro  whose  feldspar  is  altered  to  saussurite. 
See  p.  103. 

Saxonite,  Wadsworth's  name  for  peridotites  consisting  of  enstatite  or 
bronzite  and  olivine.  Synonym  of  harzburgite,  but  saxonite  has  priority. 
Lithological  Studies  1884,  p.  85. 

Schalstein,  an  old  name  for  a  more  or  less  metamorphosed  diabase- 
tuff. 

Schiller-fels,  enstatite  or  bronzite  peridotite  with  poicilitic  pyroxenes. 
Orthorhombic  pyroxenes  possess  the  poicilitic  texture  to  a  peculiar  de- 
gree, and  especially  when  more  or  less  altered  to  bastite,  and  the  term 
schiller,  which  expresses  this,  is  especially  applied  to  them. 

Schillerisation,  Judd's  name  for  the  process  of  producing  poicilitic 
texture  by  the  development  of  inclusions  and  cavities  along  particular 
crystal  planes.  The  cavities  are  largely  produced  by  solution,  somewhat 
as  are  etch  figures,  and  are  afterwards  filled  by  infiltration.  Quart.  Jour. 
Geol.  Soc.,  1885,  383;  1886,  82. 
"  Schist,  thinly  laminated  metamorphic  rocks  that  split  more  or  less 

readily  along  certain  planes  approximately  parallel.  See  p.  99. 
r  Schlieren,  a  useful  German  term,  largely  adoped  into  English,  for 
those  smaller  portions  of  many  igneous  rocks  which  are  strongly  con- 
trasted with  the  general  mass,  but  which  shade  insensibly  into  it.  Thus 
portions  of  granite  are  met,  much  richer  in  biotite  and  hornblende  than 
the  normal  rock,  or  much  more  coarsely  crystalline.  Pegmatite  streaV  s 
occur  and  other  differentiations  of  the  original  magma.  Several  different 
varieties  may  be  made,  for  a  discussion  of  which  see  Zirkel's  Lehrbuch 
der  Petrographie,  I.,  787,  1893. 

Schorl,  an  old  name  for  tourmaline,  still  sometimes  used  in  names  of 
rocks. 
f    Scoria,  coarse,  cellular  lava,  usually  of  basic  varieties. 


GLOSSARY.  163 

Scyelite,  Judd's  name  for  a  rock,  related  to  the  peridotites,  that  oc- 
curs near  Loch  Skye,  in  Scotland.  Its  principal  mineral  is  green  horn- 
blende, presumably  secondary  after  augite ;  with  it  is  serpentine,  sup- 
posed to  be  derived  from  olivine,  and  a  bleached  biotite.  See  Q.  J.  G. 
S.,  1885,  401. 

Secondary,  a  term  used  both  for  rocks  and  minerals  that  are  derived 
from  other  rocks  or  minerals,  such  as  sandstone,  clay,  or  other  sedi- 
ments ;  chlorite  from  augite,  etc. 

Sedimentary,  rocks  whose  components  have  been  deposited  from 
suspension  in  water.  See  p.  60. 

Selagite,  a  name  of  Hauy's  for  a  rock  consisting  of  mica  dissemi- 
nated through  an  intimate  mixture  of  amphibole  and  feldspar,  but  it  has 
been  since  applied  to  so  many  different  rocks  as  to  be  valueless. 

Selen elite,  Wadsworth's  name  for  rocks  composed  of  gypsum  or  an- 
hydrite. Kept.  State  Geol.  of  Mich.,  1891-92,  p.  93.* 

Septaria,  literally  little  walls,  a  name  applied  to  concretions,  largely 
of  argillaceous  material  which  are  traversed  by  cracks.  The  cracks  are 
filled  as  a  rule  with  calcite  or  quartz,  affording  an  intersecting  network 
from  which  weathering  may  have  removed  the  original,  included,  argil- 
laceous matter. 

Sericite-schist,  mica-schist  whose  mica  is  sericite.  See  p.  100. 
Sericite  is  also  used  as  a  prefix  to  many  names  of  metamorphic  rocks 
containing  the  mineral. 

Serpentine,  a  metamorphic  rock  consisting  chiefly  of  the  mineral  ser- 
pentine. See  p.  113. 

Shastalite,  Wadsworth's  name  for  unaltered  glassy  forms  of  andesite. 
Rept.  of  Mich.  State  Geol.,  1891-92,  p.  97. 

Shonkinite,  a  name  given  by  Weed  and  Pirsson  to  a  rock  from  the 
High  wood  Mountains,  Mont.,  which  they  define  as  "  a  granular,  plutonic 
rock  consisting  of  essential  augite  and  orthoclase,  and  thereby  related  to 
the  syenite  family.  It  may  be  with  or  without  olivine,  and  accessory 
nepheline,  sodalite,  etc.,  may  be  present  in  small  quantities."  Bull, 
Geol.  Soc.  Amer.,  VL,  415,  1895.  See  Anal.  7,  p.  34.  Later  they 
state  that  augite  should  exceed  orthoclase.  Amer.  Jour.  Sci.,  Dec.,  1895, 
p.  479. 

Shoshonite,  a  general  name  proposed  by  Iddings  for  a  group  of  igne- 
ous rocks  in  the  eastern  portion  of  the  Yellowstone  Park.  They  are  por- 
phyritic  in  texture,  with  phenocrysts  of  labradorite,  augite  and  olivine, 
in  a  groundmass  that  is  glassy  or  crystalline ;  in  the  latter  case  orthoclase 
and  leucite,  alone  or  together,  are  developed.  Chemically  they  range, 
SiO2,  50-56;  A12O3,  17-19.7;  CaO,  8-4.3;  MgO,  4.4-2.5;  Na2O3, 
3-3.9  ;  K2O,  3.4-4.4.  The  rocks  are  to  be  considered  in  connection  with 
absarokite  and  banakite.  Jour,  of  Geol.  III.,  937. 


1 64  A   HANDBOOK  OF  ROCKS. 

Siderolite,  as  used  by  Fletcher  and  generally  in  English,  a  name 
for  meteorites  that  are  partly  metallic  iron  and  partly  silicates.  As  used 
by  others  it  is  applied  to  more  purely  metallic  ones. 

Sideromelane,  von  Waltershausen's  name  for  a  basaltic  glass  from 
the  palagonite  tuffs  of  Sicily.  Vulk.  Gest.  v.  Sicilien  und  Island,  202, 

1853.- 

Silicalite,  Wadsworth's  name  for  rocks  composed  of  silica,  such  as 
diatomaceous  earth,  tripoli,  quartz,  lydite,  jasper,  etc.  Kept.  State  Geol. 
Mich.,  1891-92,  p.  92. 

Silicification,  the  entire  or  partial  replacement  of  rocks  and  fossils 
with  silica,  either  as  quartz,  chalcedony  or  opal. 

Sillite,  Giimbel's  name  for  a  rock  from  Sillberg,  in  the  Bavarian  Alps, 
variously  referred  by  others  to  gabbro,  diabase,  mica-syenite  and  mica- 
diorite.  Beschr,  der  bay.  Alpen,  184,  1861. 

r-       Sills,  an  English  name  for  an  intruded  sheet  of  igneous  rock. 
^       Silt,  a  general  name  for  the  muddy  deposit  of  fine  sediment  in  bays 
or  harbors,    and  one  much  employed  in  connection  with  engineering 
enterprises. 

Sinaite,  an  alliterative  substitute  for  syenite  proposed  by  Rozieres  be- 
cause on  Mt.  Sinai  true,  quartzless  syenites  occur,  whereas  at  Syene  the 
rock  is  a  hornblende-granite.     See  p.  35. 
/£"      Slickensides,  polished  surfaces  along  faults,  or  fractures  produced  by 

the  rubbing  of  the  walls  upon  each  other  during  movement. 
r       Soapstone,  metamorphic  rocks,  consisting  chiefly  of  talc.    See -p.  114. 

Soda -granite,  granites  especially  rich  in  soda,  or  whose  soda  exceeds 
the  potash.  Compare  analyses,  p.  30.  See  natron-granite. 

Sodalite-syenites,  syenites  rich  in  sodalite ;  close  relatives  of  nephe- 
line  syenites.  See  anal.  5,  p.  34.  Sodalite-trachytes  also  occur. 

Soil,  surface  earth  mixed  with  the  results  of  the  decay  of  vegetable  or 
animal  matter,  so  as  usually  to  have  a  dark  color. 

Solvsbergite,  Brogger's  name  for  quartzless  or  quartz-poor  grorudites, 
that  is,  medium  to  finely  crystalline  dike  rocks,  with  prevailing  alkali- 
feldspar  (mostly  albite  and  microcline)  with  aegirine,  or  in  the  basic 
varieties  with  hornblende  (kataforite),  sometimes  also  with  a  peculiar 
mica.  In  the  most  basic  members  quartz  entirely  fails  and  nepheline  ap- 
pears. (Die  Eruptivgesteine  des  Kristianiagebietes,  I.,  67.) 

Sondalite,  a  name  proposed  by  Stache  and  von  John  for  a  meta- 
morphic rock  consisting  of  cordierite,  quartz,  garnet,  tourmaline  and 
cyanite.  Jahrb.  d.  k.  k.  g.  Reichsanst,  1877,  194. 

Sordawalite,  an  old  name  for  the  glassy  salbands  of  small  diabase 
dikes  that  were  regarded  as  a  mineral.  It  is  derived  from  Sordawalar,  a 
locality  in  Finland.  Compare  wichtisite. 

Spheroidal,  a  descriptive  term  applied  to  igneous  rocks  that  break  up 


GLOSSARY.  165 

on  cooling  into  spheroidal  masses,  analogous  to  basaltic  columns ;   also 
used  as  a  synonym  of  orbicular  as  applied  to  certain  granites. 

Spherulites,  rounded  aggregates  or  rosettes,  large  or  small,  of  acicu- 
lar  crystals  that  radiate  from  a  centre.  They  are  chiefly  met  in  the 
microscopic  study  of  acidic,  volcanic  rocks  and  commonly  consist  of  feld- 
spars and  quartz.  When  of  one  mineral  they  are  called  by  Rosenbusch 
sphero- crystals.  They  may  reach  large  size,  though  mostly  microscopic. 
See  p.  21. 

Spilite,  an  early  French  name  for  dense,  amygaloidal  varieties  of 
diabase. 

Spilosite,  a  spotted  contact  rock  produced  from  sliales  and  slates  by 
intrusions  of  diabase.  They  correspond  to  the  hornfels  of  granite  con- 
tacts. Zincken  in  Karsten  und  v.  Dechen's  Archiv.,  1845,  5^4- 

Stalactite,  depending  columnar  deposits,  generally  of  calcite,  formed 
on  the  roof  of  a  cavity  by  the  drip  of  mineral  solutions.  Compare  sta- 
lagmite. 

Stalagmite,  uprising  columnar  deposits,  generally  of  calcite,  formed 
on  the  floor  of  a  cavity  by  the  drip  of  mineral  solutions  from  the  roof. 
Compare  stalactite. 

Steatite,  soapstone,  talc  rocks. 

Structure,  used  generally  in  America  for  the  larger  physical  features 
of  rocks, as  against  texture,  which  is  applied  to  the  smaller  ones.  See  p. 
13.  Many,  however,  employ  them  interchangeably.  Compare  also  pet- 
rical  and  lithical. 

Stylolite,  small  columnar  developments  in  limestones  or  other  cal- 
careous rocks  that  run  across  the  stratification.  They  appear  to  have 
been  caused  by  some  unequal  distribution  of  pressure  in  consolidation,  or 
by  a  capping  fossil,  as  against  the  surrounding  rock. 

Subsoil,  the  layer  of  more  or  less  decomposed  and  loose  fragments  of 
country  rock  that  lies  between  the  soil  and  the  bed  rock  in  regions  not 
covered  by  transported  soils. 

Suldenite,  a  name  given  by  Stache  and  von  John  to  gray  acidic,  an- 
desitic  porphyrites  in  the  eastern  Alps.  The  range  from  54-62  SiO2  and 
have  in  the  prevailing  gray  groundmass  phenocrysts  of  hornblende,  pla- 
gioclase,  a  little  orthoclase  and  accessory  augite,  biotite  and  quartz. 
Compare  ortlerite. 

Surficial,  a  general  name  lately  introduced  by  the  U.  S.  Geological 
Survey,  for  the  untransported  surface,  alteration  products  of  igneous 
rocks. 

Sussexite,  a  special  name  suggested  by  Brogger  for  the  eleolite  por- 
phyry, originally  described  by  Kemp,  from  Beemerville,  Sussex  Co.,  N. 
J.  Die  Eruptivgesteine  des  Kristianiagebietes,  1895.  The  name  was, 
however,  applied  years  ago  to  a  borooilicate  from  Franklin  Furnace,  N.  J. 


1 66  A   HANDBOOK  OF  ROCKS. 

Syenite,  granitoid  rocks  consisting  in  typical  instances  of  orthoclase 
and  hornblende.  In  mica-diorites,  biotite  replaces  hornblende.  In  augite- 
syenites  augite  does  the  same.  For  etymology  and  history  see  p.  36. 
Compare  also  laurvikite,  monzonite,  nordmarkite,  pulaskite,  sanidinite, 
shonkinite,  yogoite. 

Syssiderite,  Daubree's  name  for  those  meteorites  which  consist  of 
silicates  cemented  together  by  metallic  iron. 


Tachylyte,  Breithaupt's  name  for  a  basaltic  glass.  It  was  originally 
regarded  as  a  mineral  and  was  named  from  two  Greek  words  suggested 
by  its  quick  and  easy  fusibility.  See  analysis  15.  p.  20,  and  description 
p.  21.  Kastner's  Archiv.  fur  die  gesammte  Naturlehre,  VII.  ,  112, 
1826.  Compare  hyalomelane. 

Taconyte,  a  name  proposed  by  H.  V.  Winchell  for  the  cherty  or  jas- 
pery  but  at  times  calcareous  or  more  or  less  quartzitic  rock  that  encloses 
the  soft  hematites  of  the  Mesabi  Range,  Minn.  They  are  regarded  as 
in  large  part  altered  greensands  by  J.  E.  Spurr.  The  term  is  current  in 
the  Mesabi  iron  range.  XX.  Ann.  Kept.  Minn.  Geol.  Survey,  124. 
The  name  is  derived  from  Taconic,  E.  Emmons'  rejected  geological 
system. 

Talc-schist,  schistose  rocks  consisting  chiefly  of  talc  and  quartz. 
See  p:  104.  Talc  is  also  prefixed  to  several  other  rock  names. 

Taxite,  Loewinson-Lessing's  name  for  lavas  that  on  crystallizing  have 
broken  up  into  contrasted  aggregates  of  minerals  so  as  to  present  an  ap- 
parent clastic  structure  —  either  banded,  /.  e.,  eutaxitic,  or  brecciated, 
i.  e.,  ataxitic.  Bull.  Soc.  Belg.  Geol.,  V.,  104,  1891. 

Tephrite,  basaltic  rocks  containing  lime-soda  feldspar,  nepheline, 
augite  and  basis.  Leucite-tephrites  have  leucite  in  place  of  nepheline, 
and  some  tephrites  have  both.  Tephrites  differ  from  basanites  in  lack- 
ing olivine.  The  name  is  from  the  Greek  for  "  ashen,"  alluding  to  the 
color.  It  is  an  adaptation  of  an  old  form  tephrine.  Neues  Jahrb.,  1865,  663. 

Teschenite,  a  name  given  in  1861  by  Hohenegger  to  a  group  of 
intrusive  rocks  in  the  cretaceous  strata  near  Teschen,  Austrian  Silesia. 
They  have,  however,  been  since  shown  to  embrace  such  a  variety  of  types 
that  the  name  has  little  value,  but  as  analcite  occurs  quite  constantly  in 
most  of  them,  many  still  use  the  term  for  diabasic  rocks  with  this  mineral. 

Texture.     See  structure  and  also  p.  13. 

Theralite,  granitoid  rocks  consisting  essentially  of  plagioclase,  nephe- 
line and  augite,  with  the  common  accessories.  They  were  first  discovered 
by  J.  E.  Wolff  in  the  Crazy  Mountains,  Montana.  They  were  previously 
prophetically  named  by  Rosenbusch  from  the  Greek  to  seek  eagerly,  be- 


. 
^ 


GLOSSARY.  I<  1 6; 

cause  this  mineralogical  and  textural  aggregate  was  believed  to  exist  be- 
fore it  was  actually  discovered.  A  spelling  therolite  is  also  advocated. 

Tholeite,  Rosenbusch's  name  for  augite-porphyrites,  which,  aside  from 
the  usual  phenocrysts,  have  a  groundmass,  with  but  one  generation  of 
crystals  and  witn  a  little  glassy  basis  between  them,  affording  a  texture 
called  intersertal.  Massige  Gest.,  504,  1887. 

Till,  unsorted  glacial  deposits,  consisting  of  boulders,  clay  and  sand. 

Timazite,  a  name  given  by  Breithaupt  to  certain  porphyritic  rocks  in 
the  Timok  Valley  of  Servia,  that  have  since  proved  to  be  varieties  of 
andesite  and  dacite.  Berg,  und  Hiittm.  Zeit,  1861.  51. 

Toadstone,  an  old  English  name  for  certain  intruded  sheets  of  amyg- 
daloidal  basaltic  rocks  in  the  lead  district  of  Cumberland,  England. 
Also  locally  applied  near  Boston  to  a  mottled  felsite,  apparently  spheru- 
litic. 

Tollite,  a  biotite,  hornblende,  porphyrite,  with  garnets,  that  forms 
dikes  in  mica-schist  and  gneiss  near  Meran,  in  the  Tyrol.  Pichler,  Neues 
Jahrb.  1873,  94°  • 

j  Tonalite,  a  quartz-mica-hornblende  diorite  from  near  Meran  in  the 
Tyrol.  It  was  named  by  vom  Rath,  from  Tonale,  a  place  on  Mt.  Ada- 
mello.  Zeit.  d.  d.  g.  Gesellsch.  XVI,  249,  1864.  Compare  adamellite. 

Topazfels,  a  brecciated,    contact  rock,  near  granite  contacts,  and 
formed  of  topaz,  tourmaline,  quartz  and  some  rarer  accessory  minerals, 
'ouchstone.     See  basanite. 

Tourmaline-granite,  a  variety  of  granite  with  tourmaline  as  the 
dark  silicate.  It  is  usually  due  to  fumarole  action  and  fs  developed 
on  the  borders  of  intrusions  of  normal  granites. 

Trachorheite,  a  name  proposed  by  F.  M.  Endlich  as  a  collective 
designation  for  the  four  rocks,  propylite,  andesite,  trachyte  and  rhyolite, 
as  used  by  von  Richthofen.  Hayden's  Reports,  1^73,  p.  319. 

Trachyte,  volcanic  rocks  of  porphyritic  or  felsitic  texture  consisting 
essentially  of  orthoclase  and  biotite  or  hornblende  or  augite,  one  or  more. 
See  p.  27.  It  was  formerly  used  for  both  rhyolites  and  trachytes  proper, 
or  practically  as  a  field  name  for  light  colored  lavas  and  porphyries.  As 
such  in  older  reports  it  is  to  be  understood.  Compare  also  acmite-trach- 
vtes  and  pantellerites. 

x^/  Trachytic  structure,  a  special  microscopic  name  for  those  ground- 
masses  that  are  made  up  of  rods  of  feldspar,  usually  in  flowlines,  but 
without  basis. 

.  Trap,  a  useful  field  name  for  any  dark,  finely  crystalline  igneous  rock. 
It  is  a  Swedish  name  from  the  occurrence  of  such  rocks  in  sheets  that  re- 
semble steps,  "trappar."  See  p.  56. 

Trass,  a  trachytic  tuff  from  the  Laacher  See  used  along  the  Rhine 
for  hydraulic  cement. 


A   HANDBOOK  OF  ROCKS. 

Travertine,  calcareous  tufa.  The  name  was  given  by  Naumann  and 
is  of  Italian  origin. 

Trichite,  microscopic  term  for  hair-like  crystallites ;  so  named  from 
the  Greek  for  hair. 

Tripoli,  a  name  applied  to  diatomaceous  earth  and  to  pulverulent 
silica  derived  by  the  breaking  down  of  cherts  from  some  change  not  well 
understood.  See  p.  81. 

Troctolite,  Bonney's  name  for  a  variety  of  gabbro  consisting  of 
plagioclase  and  olivine  with  very  subordinate  diallage.  The  olivine  may 
be  serpentinized.  Geol.  Magazine,  1885,  439. 

Trowlesworthite,  a  variety  of  granite  which  has  been  so  altered  by 
fumarole  action  that  it  consists  of  fluorite,  orthoclase,  tourmaline  and 
some  quartz,  the  last  named  having  been  largely  replaced  by  the  first. 
The  name  is  derived  from  an  English  locality,  and  was  given  by  Worth. 
Trans.  Roy.  Geol.  Soc.  of  Cornwall,  1884,  180.  Mineralog.  Mag., 
1884,  48. 

^      Tufa,  the  cellular  deposits  of  mineral  springs,  usually  calcareous  or 
siliceous.     See  p.  72.     Not  to  be  confused  with  tuff. 

Tuff,  the  finer,  fragmental  ejectments  from  the  explosive  eruptions  of 
volcanoes.  They  may  afterwards  be  water-sorted  or  cemented  to  firm 
rock.  Coarser  ones  are  called  volcanic  breccias,  but  in  neither  is  it  fre- 
quent to  see  much  sorting  unless  by  subsequent  erosion.  Tufa  is  also 
used  in  this  sense.' 

Typhonic  rocks,  Brogniart's  name  for  rocks  that  have  come  from  the 
depths  of  the  earth,  /.  <?.,  plutonic  and  eruptive  rocks.  Typhon  is  used 
as  a  synonym  of  boss  or  stock. 

u 

Uralite,  a  special  name  for  that  variety  of  hornblende  that  is  derived 
by  paramorphism  from  augite.  The  word  is  often  used  as  a  prefix  before 
the  names  of  those  rocks  that  contain  this  variety.  It  has  also  suggested 
various  rock  names,  such  as  proterobase,  scyelite,  etc.  The  name  is  de- 
rived from  the  original  occurrence  in  the  Urals.  (G.  Rose,  Reise  nach 
dem  Ural,  II.,  1842,  371.) 

v 

Variolite,  a  special  name  for  a  curious,  border  development  of  dia- 
base  intrusions,  which  is  a  very  dense  finely  crystalline  mass  of  rounded 
spheroids,  largely  spherulitic  in  texture.  They  give  the  rock  a  pock- 
marked aspect  and  hence  the  name,  which  is  a  very  old  one.  Pearl 
diabase  is  synonymous. 

Vintlite,  a  quartz-porphyrite  occurring  in  dikes  near  Unter-Vintl,  in 


GLOSSARY.  ^169 

the  Tyrol.    Compare  tollite  from  the  same  region.    Pichler,  Neues  Jahrb.,, 
1871,  262. 

Viridite,  a  microscopic  name  suggested  by  Vogelsang  and  formerly 
used  for  the  small  green,  chloride  scales  often  met  in  thin  sections.  As 
their  true  nature  has  now  been  determined,  they  are  generally  called 
chlorite. 

Vitro,  a  prefix  meaning  glassy  and  used  before  many  rock  names,  as 
vitrophyre,  in  order  to  indicate  a  glassy  texture. 

Vitrophyre,  Vogelsang's  name  for  quartz-porphyries  and  porphyries 
with  glassy  groundmass. 

Vogesite,  Rosenbusch's  name  for  syenitic  dikes,  in  which  the  dark 
hornblendes  or  augites  are  in  excess  over  the  light  colored  feldspars. 
Mass.  Gest.,  1887,  319.  The  name  is  derived  from  Vogesen,  the  Ger- 
man form  of  Vosges. 

Volcanic,  surface  flows  of  lava  as  distinguished  from  plutonic  rocks. 
See  p.  13. 

Volcanite,  a  name  proposed  by  W.  H.  Hobbs,  for  an  anorthoclase- 
augite  lava  with  the  chemical  composition  of  dacite.  Bull.  Geol.  Soc. 
Amer.,  V.,  598.  The  name  was  suggested  by  the  original  occurrence  on 
the  island  of  Volcano,  one  of  the  Lipari  group,  where  the  rock  occurs 
as  a  cellular  bomb. 

Volhynite,  a  porphyrite  containing  plagioclase,  hornblende  and  bio- 
tite  phenocrysts,  in  a  holocrystalline  groundmass  of  feldspar  and  chlorite. 
The  name  was  given  by  Ossovsky,  and  is  based  on  the  original  occurrence 
in  Volhynia.  See  Chrustschoff,  Bull.  Soc.  Min.  France,  1885,  441. 

w 

Wacke,  an  old  name  for  the  surficial  clayey  products  of  the  alteration 
of  basalt.  The  syllables  are  still  current  in  graywacke. 

Wash,  a  miner's  term  in  the  West  for  loose  surface  deposits  of  sand, 
gravel,  boulders,  etc. 

Websterite,  a  name  proposed  by  G.  H.  Williams  for  the  pyroxenites 
near  Webster,  N.  C.,  that  consist  of  diopside  and  bronzite,  with  the  latter 
porphyritically  developed.  Amer.  Geol.,  VI.,  35,  1890.  The  name 
websterite  had  been  previously  used  by  A.  Brogniart  in  1822  for  alumi- 
nite;  Hauy's  Mineralogie,  II.  125. 

[/' Wehrlite,  a  name  originally  suggested  by  von  Kobell  for  what  was 
supposed  to  be  a  simple  mineral,  but  which  proved  to  be  a  peridotite  con- 
sisting of  olivine  and  diallage. 

Weiselbergite,  Rosenbusch's  name  for  those  augite-porphyrites 
whose  groundmass  consists  of  a  second  and  sometimes  third  generation  of 
plagioclase  rods  and  augites,  arranged  in  flow  lines  in  a  glassy  basis. 


i/o  *  A   HANDBOOK  OF  ROCKS. 

Mass.  Gest.,  501,  1887.  Wadsworth  uses  the  name  for  an  altered  ande- 
site  glass.  Rept.  of  State  Geol.  of  Mich.,  1891-92,  p.  97. 

Whinstone,  a  Scotch  name  for  basaltic  rocks. 

Wichtisite,  a  glassy  phase  of  diabase,  named  from  a  Finland  local 
ity,  Wichtis.  Compare  sordavalite. 


Xenogenites,  Posepny's  term  for  mineral  deposits  of  later  origin  than 
the  wall  rock.  The  name  means  foreigners,  and  refers  to  their  later  in- 
troduction. Compare  idiogenites.  Trans.  Amer.  Inst.  Min.  Eng., 
XXIIL,  205,  1893. 

*  fr  Xenomorphic,  Rohrbach's  textural  name  for  those  minerals  in  an 
igneous  rock  whose  boundaries  are  determined  by  their  neighbors.  Its 
antithesis  is  automorphic,  which  see.  Xenomorphic  is  synonymous 
with  allotriomorphic,  over  which  it  has  priority.  Tsch.  Mitt.,  1886,  18. 


Y 

Yogoite,  a  name  suggested  by  Weed  and  Pirsson  from  Yogo  peak,  one 
of  the  Little  Belt  Mountains,  Mont.,  for  a  syenitic  rock  composed  of 
orthoclase  and  augite  in  about  equal  amounts.  See  also  sanidinite  and 
shonkinite.  Amer.  Jour.  Sci.  Dec.,  18^5,  473-479. 

z 

Zircon-syenite,  a  name  originally  given  by  Hausmann  to  certain  Nor- 
wegian nepheline  syenites  which  were  rich  in  zircons.  Later  it  was 
practically  used  as  a  synonym  of  nepheline  syenite,  but  is  now  obsolete. 

Zirkelite,  a  name  proposed  by  Wadsworth  in  1887  to  designate  altered 
basaltic  glasses,  in  distinction  from  their  unaltered  or  tachylitic  state. 
Geol.  Surv.  Minn.  Bull.  2,  1887,  p.  30. 

Zobtenite,  Roth's  name  for  metamorphic  rocks  with  the  composition 
of  gabbros,  /.  ^.,  rocks  not  certainly  igneous.  The  name  is  derived  from 
the  Zobtenberg,  a  Silesian  mountain.  Sitz.  Berl.  Akad.,  1887,  6n. 

Zonal-structure,  a  term  especially  used  in  microscopic  work  to  de- 
scribe those  minerals  whose  cross-sections  show  their  successive,  con- 
centric layers  of  growth. 

Z witter,  a  Saxon  miner's  term  for  a  variety  of  greisen.  Only  of  sig- 
nificance in  connection  with  tin  ores. 


INDEX. 


NOTE., — The  index  only  concerns  the  main  portion  of  the  book  and  not  the  Glossary, 
Attention  may  be  called  to  the  latter  as  embracing  many  rocks  not  otherwise  men- 
tioned. 

Basalt,  defined,  .........  43 

Alterations,  Metamorphism,  ...  45 

Distribution, 45 

Mineralogical  Composition,    ...  43 

Tuffs, 45 

Varieties, 43 

Basanite, 44 

Bathylite  defined, 12 

Becker,  G.  F.,  cited  on  Asperite,  .    .  41 

Metamorphism  of    Sandstone,  .    .  68 

Pseudodiorite,  Analysis,     ....  102 

Slate, 109 

California  serpentine, 114 

Saprolite, 117 

Beemerville,  N.  J.,hornfels,  ....  90 

Nepheline-syenite, 36 

Binary  granite, 31 

Biotite, 8 

Bituminous   Coal, 7^ 

Bosses,   defined, 12 

Bostonite,  defined,  . 27 

Brazil,  Weathered  rocks  of,  ....  116 

Breccias, 59 

Bronzite, 7 

Bytownite, 5 


39 


Accessory  Minerals, 9 

Acmite,     . 

Actinolite,     . , 

Aegirine, 

Alabaster, , 

Amphibole,       , 

Amphibolites,  see  hornblende-schists,  101 

Andalusite, 

Andesine, 

Andesites,    analyses,    Eureka,    Nev.; 

Yellowstone    Park;    Rosita   Hills, 

Colo.;    Lassen's  Peak,   Cal.;    Mt. 

Rainier,  Wash.;  Red  Bluff,  Mont.; 

Buffalo;      Peaks,  Col.;     Colombia, 

S.  A. 

Defined, 40 

Anhydrite, 

Anogene, 13 

Anorthite ...5. 

Anorthoclase, \   5 

Anorthosites,  analyses, 49 

Defined,    ..." 50 

Anthophyllite, 7 

Anthracite, j6 

Apatite, 9 

Aplite 31 

Apobsidian, 21 

Aporhyolites, 24 

Aqueous  Rocks, 10,  58 

Arfvedsonite,    .    .    .    \ 7 

Asperite, 41 

Atmospheric  weathering  defined,  .    .       85 

Rocks  produced  by 116-119 

Augen, 31,  52 

Augen-gneiss,       94 

Augite,      7 

Defined, 8 

Augite-andesite 4° 

Augitite,  ...      , 45 


Bar-theory  of  Rock-salt, 78 

Basalt,  Analyses,  Cinder  Cone,  Cal., 
Kilauea,  S.  I.;  Iceland;  Rio  Grande 
Canon,  N.  M.;  Dalles,  Oregon;  Eu- 
reka, Nev.;  Point  Bonita,  Cal.; 
North  Park,  Colo.;  Shoshone  Mesa, 
Nev.;  Cascade  Mtns,  Oregon;  Edge- 
combe  Island,  Alaska, 42 


Calcareous  sandstone,  analyses,  Flag- 
staff, Arizona  ;  Jordan,  Minn.,  .    .  68 
Calcareous  shale,analyses,  Mt.  Morris, 
N.  Y.;  Rochester,  N.  Y.,    .    .    .    .  68 

Calcite 9 

Calcite  anals., 7° 

Calc-schist, ,  101 

Camptonite 48 

Cancrinite,    .    .    , 37 

Carbonaceous  Organic  Rocks,   ...  76 

Metamorphism, 77 

Occurrence, 77 

Chemical  Elements  in  Rocks,    ...  3 

Chert,  analyses,  England, 74 

Joplin,  Mo.;  Galena,  Kansas ;  Bell- 
ville,  Mo.;  Seneca,  Mo.;  Newton 

Co.,  Mo., .  80 

Cherty  iron  carbonates,analyses,Minn ; 

Gogebic  Range,  Mich.,  .....  80 

Chlorite, 9 

Chlorite-schist,  analyses,  from  Klippe, 

Sweden ;    Foster  Mine,  Mich.,     .  103 

Defined, 104 


1. 72 


INDEX. 


-Chromite, ,    .         9 

Clarke,  F.  W.,  cited, 3 

Clay, 

Brick  Clay,  analyses,  Washington, 
Ind.;  Salem,  Ind.;  Plattsburg, 
N.  Y.;  La  Salle,  111.;  Rondout, 
N.  Y.;  Fisher's  Island,  N.  Y.; 
Hooversville,  Pa.; 66 

Potters'1  Clay,  Akron,  Ohio  ;  East 
Liverpool,  Ohio, 66 


/Yr<?C/tfj,Woodbridge,  N.  ].;  Chel- 


66 


tenham,  Mo.;  Woodland,  Pa., 
Residual   Clay,   Morrisville,  Ala.; 

Batesville,  Ark., 66 

Conanicut  Island,  R.  I.;  contact  met- 

amorphism, 89 

Consanguinity, 57 

Contact  Metamorphism,internal  effects  87 

External  effects,          88 

On  sedimentary  rocks 88,  91 

Zones, 88 

Cornwall,  Penn.,  magnetite,  ....  91 
Crawford  Notch,  N.  H.,  contact   met- 

amorphism,       32,  89 

Cross  W.,  cited,  .........  22 

Crugers  on  Hudson,  contracts,       .  90 

Crystalline   Limestones,   analyses   of, 

from  Brandon,  Vt;    Carrara,  Italy; 

Knoxville,  Tenn.;  Pickens  Co.,  Ga.; 

Rutland,  Vt.;  Franklin  Furnace,  N. 

].;  Tuckahoe,  N.  Y., no 

Mineralogy, in 

Varieties, Ill 

Alteration, 112 

Occurrence, 112 

Crystalline  Schists, 99 

Cyanite, 9 

Cycle  of  deposition, 61 

D 

Dacites,  analyses:  Comstock  Lode, 
Nev.;  Lassen's  Peak,  Cal.;  Yellow- 
stone Park;  Hungary;  Eureka, Nev.; 

Colombia,  S.  A., 39 

Defined 40 

Dana,  J.  D. ,  cited,  , 100 

Degeneration, 117 

Derby,    O.  A.,  cited  on  alteration  of 

rocks, 116 

Diabases,  analyses,  Diabase  Hills, 
Nev.;  Jersey  City,  N.  J.;  Lake  Sal- 
tonstall,  Conn.;  Boston,  Mass.; 
Point  Bonita,  Cal.;  Ausable  Forks, 

N.  Y., 42 

Defined, 44 

Diallage, 7 

Defined, 7,  8 

Diatomaceous   earth.    See    Infusorial 

Earth, 74 

Dikes,   defined, 12 

Diller,  J.  S.,  cited, 45 

Diopside, 7 


Diorites,  analyses,  Yellowstone  Park; 
Watab,  Minn.;  Wales;  Comstock 
Lode;  Little  Falls,  Minn.;  Custer 
Co.,  Colo.;  St.  John,  N.  B.;  Forest 

of  Dean,  N.  Y., 47 

Defined, 48 

Alteration 48 

Distribution, 48 

Dissolved  vapors, 15 

Ditroite,  Anals, 36 

Defined, 37 

Dolerite, 44 

Dolomite, 9 

As  a  Mineral, 9 

As  a  Rock, 70,  95 

Cystalline,  analyses  from  Tuckahoe, 

N.  Y.;  Inyo  Co.,  Cal , no 

Dykes,  see  dikes. 


Eclogite,  analysis,  Altenburg,  Austria,  103 

Defined, 105 

Effusive, 13 

Eleolite  porphyry,  analyses,  Beemer- 

ville,  N.  J.;  Magnet  Cone,  Ark.,    .  28 

Eleolite-syenite,see  Nepheline-syenite.  36 

Enstatite,  .    .            7 

Eolian  Rocks, 10,  58 

Eolian  sandstone 64 

Epidote, 9 

Epidote   schist,  analysis   from    South 

Mountain,  Pa., 103 

Defined, 104 

Essential  Minerals, 9 

Extrusive J3 


Feldspars,  analyses, 6 

Defined, 4 

Feldspathoids,      .    '. 4 

Defined, 6 

Felsite,      .    .    .    .  ' 24 

Felsitic  texture, 15 

FeiTomagnesian  silicates,  defined  .    .  4 

Ferruginous  organic  rocks,    ....  76 

Ferruginous  precipitates,    .....  82 

Foyaite  anals, 36 

Defined, 37 

Freshwater  Limestone,  analyses,  .    . 

Chalk  Bluffs,  Wyo.;  Henry'sForks, 

Wyo., 7° 

Friction  breccia, 59 

o 

Gabbro,  analyses,  Chateau  Richer, 
Can.;  Montrose  Pt.,  N.  Y.;  Crotosn 
River,  N.  Y.;  Mt.  Marcy,  N.  Y.; 
Nain,  Labrador;  Iron  Mt.,  Wyo.; 
Duluth,  Minn.;  Baltimore,  Md.; 
Adirondacks,  N.  Y.;  N.  W.  Minn.,  49 
Defined, 50 


INDEX. 


173 


Mineralogy,      .......    .    .  50 

Varieties,      ..........  50 

Alteration,    ..........  52 

Metamorphism,    .......    .  52 

Distribution,     ..........  52 

Garnet,     ............  8 

Generations  of  Minerals,    .....  17 

Geyserite,  analysis,  Yellowstone  Park,  80 

Glasses,     ............  20 

Varieties,  ...........  21 

Relationships,  .........  22 

Geological  occurrence,    .....  22 

Alteration,    ..........  22 

Distribution,     .........  22 

Glauconite,  ...........  69 

Glaucophane-schist,      analysis      from 

Monte  Diablo,  Cal.,     ......  103 

Defined,    ............  105 

Gneisses,  ............  95 

Defined  and  classified,  .....  95,  96 

Analyses  from  Black  Hills;  Munson, 
Mass.;  Iron  Mtn.,  Wyo.;  Trembling 
Mtn.,  Quebec;  St.  Jean  cle  Matha, 
Quebec;  New  York  City;  Rawdon, 

Quebec,     ..*...    ......  97 

Alteration,    ..........  98 

Distribution,     .........  98 

Gordon,  C.  H.,  cited,    ......  95,  96 

Granite,   analyses,   Green's  Landing, 
Me.;  Peterhead,  Scotland;  Westerly, 
R.  I.;  Stony  Point,  Conn.;  Crawfcrd 
Notch,  N.  H.;  Cottonwood  Canon, 
Utah;  Chester,  Mass.;  Raleigh,  N. 
C.;  Eureka  Dist.,Nev.;  Kekequabic 
Lake,   Minn.,    Rowlandville,    Md.; 

Yosemite,      ..........  30 

Defined,    ...........  31 

Alteration,  Metamorphism,     ...  33 

Distribution,     .........  34 

Geological  Occurrence,  .....  32 

Mineralological   Composition,    .    .  31 

Relationships,  .........  32 

Uses,     ............  33 

Varieties,  ...........  31 

Granite  porphyry,  ......  24,  31,  32 

Granitoid  texture,    ........  13 

Grano-diorites,     .........  32 

Granulite,    *  ...........  98 

Graphic  granite,  .........  31 

Graphite  schist,    .........  105 

Gravels,    ............  62 

Greensand,  ...........  69 

Greenstone,  ...........  44 

Greisen,    .........    ...  32 

Groundmass,    ..........  13 

Gypsum,  ............  9,  78 


Halite,  .............         9 

Hawes,  G.  W.,  cited,  ......    32,  89 

Hematite,    ...........         9 

Hoboken,  N.  J.,  contacts,  .....      89 


Hornblende, 7,  8- 

Hornblende-granite,    .......  31 

Hornblende-schist, 101. 

Analyses  from  Grand  Rapids,Mich.; 
Knoxville,  Cal.;  Lower  Quinnesec 
Basin  and  Falls,  Wis ;  Cleveland 

Mine,Mich., 102 

Mineralogy  and  varieties,   ....  102 

Alteration,       103 

Occurrence, 103 

Hornblendite, 51 

Hornfels, 88- 

Hyalomelane,  analysis, 20 

Hydromica  schist,   . 100 

Hypersthene, ...  7- 

Hypersthene-andesite, 40 

Hypersthene-fels 51. 

Hypersthene  rock, 51, 


Iddings,  J.  P.,  cited,      ......  .22,  36- 

Ilmenite,  ............  9, 

Infusorial  earth,  analyses,  Little  Truc- 

kee  River,  Nev.;  Fossil  Hill,  Nev.; 

Richmond,  Va.,  ........  74 

Intratelluric  ..........  15 

Itacolumite  (See  especially  p.  107),  65. 


Kaolin,    .    .     .........  9,  66 

Kaolin,  anals,  ..........       66 

Keratophyre,  ..........       27 

Kimberlite(see  errata),  ......       51 

Knotty  schists,  .........       89 

Knotty  slates,  ..........      89. 


Labradorite, 5 

Labradorite-rock, 51 

Laccolite,  defined, 12 

Laterite 116,  117,. 

Laurdalite,  anals, 36 

Lavas, 19^ 

Leucite, 6 

Leucite-basalt, .  44 

Leucite-basanite, .  44 

Leucite-phonolite,  anals,  Rieden,  Ger- 
many,   28- 

Defined, 29 

Leucite-syenite,  anals, 36 

Leucite-tephrite, 44^ 

Leucitophyre, 29""""" 

Lignite, .76,  77 

Limburgite,  analyses,  Bozeman.Mont.; 

Palma,  Italy,        42 

Defined, 45, 

Limestone,  analyses,  Adams,  Mass.; 
Bedford,  Ind.;  Solenhofen,  Ger- 
many; Hudson,  N.  Y.;  Point  Pleas- 
ant, Ohio;  Bonne  Terre,  Mo; 


INDEX. 


Chicago,  111.,    .........  70 

Metamorphism,    ........  73 

Mineralogy  ..........  72 

Occurrence,      ...    ......  73 

Origin,      ...........  71 

Varieties,  ...........  72 

Limestones,  Crystalline,    .....  no 

Limonite,      ...........  9 

Liparite,  defined,    ........  24 

Litchfieldite,  anals.,    .......  36 

Defined,    .....    ......  37 

Lithophysae,   .....    .    .  21,  25 

Living  organisms,  analyses  of  calca- 

reous parts,  ..........  70 

Loess,    .............  65 

Luxulianite,      ..........  32 

Lyell,  Sir  Charles,  cited  on  Metamor- 
phism ............  84,  85 


Magnesian  Limestone,    ......  72 

Magnetite,    ...........  9 

Malacolite,    .    ..........  7 

Marble,  see  Crystalline  Limestone.  1  1  1 
Marls,   analyses,   Hop  Brook,  N.  J.; 
Red  Bank,  N.  J.;  Bowling  Green, 

Ky.,  .............  68 

Melilite  .............  6 

Melilite  basalt,     .........  45 

Merrill,  G.  P.,  cited  on, 

Alteration  of  Granite,      .....  33 

Ophicalcite  ...........  114 

Degeneration,       ........  117 

Metamorphic  Rocks,     ......  11,84 

Determination  of,     .......  119 

Metamorphism,  defined,      .....  84 

Contact  metamorphism  defined,     .  85 
Described,      .........  85-87 

Rocks  produced  by,  ......  87-92 

Regional  metamorphism,   ....  93 

Meteorites,    ...........  53 

Miarolitic,     ...........  13 

Mica,     ............    ,  8 

Mica-andesite,  ..........  40 

Mica    schists,   analyses   from    Monte 
Rosa,  Switzerland;   Zermatt,  Swit- 
zerland;   Brixen,   Tyrol;    Meissen, 
Saxony  ;    Crugers,    N.  Y.  ;      New 
Hampshire;  Messina,  Sicily;  Wis- 

consin,      ...........  99 

Mineralogical  composition,     .    .    .  100 

Varieties  ............  100 

Alteration,    ..........  101 

Distribution,     .        .......  101 

Microcline,    .......    '.    .    .    .  5 

Mineralizers,     ........    .    .  15 

Minette,  analysis,  from  Rhode  Island,  34 
Mount  Willard,  N.  H.,     .....  32,  89 

Muscovite,    ...........  8 


Nepheline, 

Nepheline-basalt,analysis,  Pilot  Knob, 

Texas, 

Defined,    ..........', 

Nepheline-syenite,     analyses,     Litch- 

field,  Me.;  Fourche  Mtn.,  Ark.;  Red 

Mtn.,  N.  H  ;  Ditro,  Hungary;  Foya, 

Portugal;  Sao  Paulo,  Brazil;  Lund, 

Norway;  Beemerville,  N.  J.,  . 

Mineralogy, 

Relationships, 

Alteration, 

Distribution, 

Nevadite, 

Norite,  analysis, 

Defined,    ........... 

Novaculite,  analysis,    . 

Defined, 


42 
43 


36 
36 

37 
37 
37 
24 
49 
51 

8 


N 
Necks,  defined,    .  12 


O 

Obsidian,     analyses,   Tewan     Mtns.; 
Yellowstone     Park;    Mono    Lake; 

Lipari  Island;  Clear  Lake,  Cal.,    .  20 

Defined, 21 

Obsidian  Cliff,  cited, 14 

Ochsenius,  cited, 78 

Oligoclase, 5 

Olivine, 8 

Olivine-diabase, 44 

Ophicalcites, 112 

Analyses     from    Oxford,     Quebec; 

Brompton  Lake,  Quebec,  ....  112 

Mineralogy, 113,  114 

Alteration, 115 

Distribution, 115 

Orbicular  granite, 32 

Organic  Remains  not  Limestones,  .  74 

Orthoclase, 4 


Pantellerites, 24,  27 

Paramorphism, 45 

Pearlite  orPerlite,  analyses,  Hungary; 

Eureka,  Nev., 20 

Defined, 21 

Peat, 76,  77 

Pegmatite 31 

Pele's  Hair,  anals, 20 

Peridotite,  analyses,  Montrose  Point, 
N.  Y.;CusterCo.,  Colo.;  Baltimore, 
Md.;Dewitt,  N.Y.;  Crittenden  Co., 
Ky.;  Elliott  Co.,  Ky.,  .  ....  49 

Defined, 51 

(For  varieties  see  Glossary.) 

Petrographic  Provinces, 57 

Phanerocryst, 13 

Phenocryst, 13 

Phlogopite, 8 

Phonolites,  analyses,  Devil's  Tower, 
Wyo.;  El  Paso  Co.,  Col.;  Fernando 
de  Noronha,  Brazil;  Zittan,  Saxony; 
Wolf  Rock,  England, 28 


INDEX. 


175 


Defined, 29 

Mineralogy, 29 

Relationships,       29 

Alteration, 29 

Tuffs, 30 

Phonolite-obsidian,  analysis,  ....  20 

Phthanites, 68 

Phyllite 101,  109 

Pirsson,  L.  V.,  cited, 89 

Pitchstone,  analyses,     Meissen;  Silver 

Cliff,  Colo., 20 

Defined, 21 

Plagioclases, 4 

Defined, 5 

List  of, 5 

Plutonic, 13 

Porphy  rites, 40 

Porphyritic  texture, 13 

Porphyry  defined, 27 

Precipitates  from  Solution,    ....  77 

Pressure  as  influencing  texture,  .    .  15 

Primai-y  Minerals, IO 

Principles  underlying   the  Classifica- 
tion of  Rocks, IO 

Propylite  (see  errata  and  glossary)  .  41 

Pumice,  analyses, 2O 

Pyrite, 9 

Pyroxenes, 7 

Tabulated, '  7 

Pyroxenite,  analyses,  Webster,  N.  C.; 
Baltimore,    Md.;    Meadow    Creek, 

Mont.;  Montrose  Point,  N.  Y.,  .    .  49 

Defined, 51 

Pyrrhotite, 9 


Quartzites,  analyses  from,  Chickies 
Station,  Pa.;  Quarry  Mtn.,  Ark.; 

Pipestone,  Minn.; 106 

Mineralogy, 106 

Varieties,      106 

Alteration 107 

Distribution, 107 

See  also,   .    .        65 

Quartzkeratophyre, 24 

Quartz-porphyry,  analyses,  Leadville, 
Colo.;  Flagstaff  Hill,  Colo.;  Upper 
Quinnesec  Falls,  Mich.;  Waterville, 

N.  H., 23 

Defined,    ....            24 

Quartz-trachyte,  defined, 24 

R 

Regional  Metamorphism,  defined,    .      85 

Described, 93,  94 

Rocks  produced  by, 95,  115 

Rhyolites,  analyses,  Berkeley,  Cal.; 
Iceland;  Wales;  Silver  Cliff;  Eu- 
reka, Nev.;  Washoe  Dist,  Nev-.; 
Ponza,  Italy;  Yellowstone  Park; 
Lassen's  Peak,  Cali.;  Hungary,  .  23 


See  also  Quartz-porphyry,  ....  23 

Defined, 

Mineralogy, 25 

Varieties,      23 

Alteration,    .                                   2c,  117 

Tuffs,             ....'....'..  26 

Distribution, 25 

Rock,  definition  of, i 

Solidity  of, 2 

Rock  salt, 78 

Rosenbusch,  cited, 89 


St.  John,  N.  B.,  contacts, 90 

Sand,  analysis,  Chesire,  Mass.,  .  .  63 
Sandstone,  analyses,  Juniata  Valley, 
Penna.;  Crystal  City,  Mo.;  Rock- 
port.  Ark.;  Glencoe,  Col.;  Portage 
Lake,  Mich.;  Cleveland,  O.;  Dor- 
chester, N.  B.;  Portland,  Conn.,  .  63 

Defined, 64 

Mineralogy,      69 

Varieties, 69 

Metamorphism, 69 

Occurrence,      69 

Sanidine, c 

Saprolite,      117 

Scapolite,      8 

Schists, 99 

Scorias, 21 

Secondary  Minerals, 10 

Sedimentation, 60 

Serpentine  (Mineral), 9 

Serpentines, 112 

Analyses,  from  Webster,  N.  C.; 
Montville,  N.  J.;  New  Idria,  Cal.; 
Syracuse,  N.  Y.;  Dublin,  Md.; 
Presq'  Isle,  Mich.;  Monte  Diablo, 

Cal.;       112 

Mineralogy, 113,  114 

Alteration, 115 

Distribution, 115 

Shale,  analyses,(for  No.  I  see  errata); 
Haydenville,  O.;  Hornellsville,  N. 
Y.;  Kansas  City,  Mo.;  Sharon,  Pa.; 
Leavenworth,  Kan.;  Clinton,  Ind.,  66 

Defined, 67 

Sheet,  defined, 12 

Shonkinite*  anals, 34 

Siderite, 9 

Siliceous  Limestone,  Chicago,  111.,    .       70 
Siliceous  oolite,  analysis,  Center  Co., 

Pa., 80 

Siliceous  sinter, 74 

Analysis,  Yellowstone  Park,  ...       74 

Sillimanite, 9 

Slates,  analyses  from  Llanberis,Wales ; 
Etchemin  River,  N.  B.;  Westbury, 
Can.;  Lehesten,  Germany,  Hen- 
singerville,  Pa.;  Wales;  Lancaster 
Co.,  Pa.;  Angers,  France ;  Peach 
Bottom,  Pa.;  Kingsey,  Quebec,  .  .  107 


1 76 


INDEX. 


Mineralogy,      108 

Varieties,      108 

Alteration, no 

Distribution, no 

Smyth,  C.  H.,  Jr.,  cited, 90 

Smyth,  C.    H.,    Jr.,  cited    on    Gou- 

verneur  talc, 115 

Soapstones, 112 

Analysis  from  Webster,  N.  C,  .    .  112 

Mineralogy 113,  114 

Alteration, 115 

Distribution, 115 

Sodalite-syenite,  analysis,  Highwood 

Mtns., 34 

Sorby,  H.  C.,  cited  on  slaty  cleavage,  109 

Specific  gravity  of  rocks, 16 

Spheroidal  granite, 32 

Spherulites,       21 

Staurolite, 9 

Structure,      13 

Syenite, 31 

Syenite,  analyses,  Fourche  Mtn.,  Ark.; 
Plauen,  Saxony ;   Custer  Co.,  Col.; 

Biella,  Piedmont, 34 

Defined, 35 

Alteration 36 

Distribution, 36 

Geological  occurrence, 35 

Mineralogical  composition,     ...  35 

Relationships, 35 

Varieties 35 

Surficial  rocks, 117 


Tabulation  of  Igneous  Rocks,   ...  18 

Tachylyte,  anals., 20 

Talc, 9 

Analysis  from  Gouverneur,  N.Y.;  .  112 
Talc-schist,    analyses    from    Fahlun, 

Sweden;    Gastein,  Austria,   .    .    .  103 

Defined 104 

Talus-breccias, 59 

Tephrite, 44 


Textures, 13 

Development  of, 14 

Theralite, 37 

Theralite,  defined, 52 

Tinguaite, 29 

Titanite, 8 

Tonalite  (see  errata  and  glossary),  .  48 

Topaz, 8 

Tourmaline,              8 

Tourmaline  granite, 32 

Trachytes,  analyses,  Game  Ridge, 
Colo.;  Drachenfels;  Lake  Cham- 
plain;  Crazy  Mtns.,  Mont.;  Ischia, 

Italy, 2§ 

Defined, 27 

Alteration, 27 

Distribution, 27 

Mineralogical  Composition,    ...  27 

Relationship,  27 

Tuffs, 28 

Varieties,                  27 

Trap, 44 

Travertine,  Yellowstone  Park,  ...  70 

Tremolite, 7 

Tripoli 75 

Analyses,      80 

Tyndall  cited  on  slaty  cleavage,     .    .  108 


Vesuvius,  ejected  bombs, 91 


w 

Wads  worth,  M.  E.,  cited, 


119 


Waterlime,   Coplay,  Pa.;    Rosendale, 

N.  Y.; 70 

Weathering  of  Rocks, 116 

Websterite  analysis  (see  glossary),   .  49 

Weed  and  Pirsson  cited,     ..:...  35 


Zeolites, 
Zircon,  . 


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